Honeywell room thermostatThermostat Won't Turn ON
Diagnostic FAQs for a room thermostat that does not turn ON the heat or A/C

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FAQs on how to diagnose & fix a room thermostat that that won't turn the heating or cooling system ON:

Thermostat ON diagnostic questions help diagnose and fix hermostat wiring problems that cause a heating system or cooling system to stay OFF when it should turn ON.

This room thermostat diagnosis and repair article series provides a sequence of diagnostic steps that determine if the problem is at the thermostat, the thermostat wiring, or at other heating or cooling system controls.

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Thermostat Troubleshooting FAQs: heating or cooling system won't turn ON

These questions & answers on fixing a thermostat that won't turn on heating or air conditioning were posted originally at THERMOSTAT WON'T TURN ON - be sure to see that diagnostic article.

But if it's not a thermostat problem see HEAT WON'T TURN ON

On 2017 10 29 Mod said: the heater won't turn on and I don't think it's the thermostat

Betty, please see the diagnostic suggestions starting at HEAT WON'T TURN ON and let me know what further help you need.

On 2017-10-29 by Betty

Me again,I wouldn't bother you tonight except it's cold. Thank you for help !

My heater won't come on. I changed batteries, there's a reset spot...what should I do? The brand is Hunter,digital.

On 2017-10-28 by Ronald kuhaneck

My Yoak 5 ton turns off on

On 2017-10-15 by Jim

I recently bought a Honeywell RTH9850WF. I have a C wire, and have confirmed 24VAC at the thermostat. When I power up the system I get a response “click” at the thermostat but no reaction from my boiler. When I hook up my old thermostat it still works fine. Is my new thermostat not functioning properly?

On 2017-08-20 by (mod) - where to find instruction manuals for heating and air conditioning equipment

Sure Vlad.

You can find Fujitsu Manuals including for the control, operation, and wiring of the ARG30ALC inverter at


On that page you'll find a link to the Fujitsu Manuals, and there you'll want

FUJITSU AIR CONDITIONER DESIGN & TECHNICAL MANUAL R410A [PDF] Inverter, Indoor & Outdoor Units including Compact Cassette AUU9-12-18RLF, Slim Duct ARU9-14-18-124RLF, Compact Wall Mount ASU7-9-12RLF, Wall Mount ASU18-24RLF includes wiring diagrams

where you'll find a live link to that file as a PDF

On 2017-08-20 by vlad

Hi Mr Ayub, Can you upload the drawings (Circuit diagram) of the O'General Duct Thermostat, Model No : ARG30ALC.

On 2017-06-16 by Joyce

The heater switches on but the temp wont increase

On 2017-06-15 by Ayub

My thermostat won't start. I suspect people doing maintenance work on the rooftop have done some damage though i am not entirely sure. I have been on the rooftop and as the building is residential there are multiple ac units for each room and i was not able to find mine. I managed to find what looked like a water pump valve with my flat number on it and right next to it there is a ac unit.

My question is,

a. How can i be sure which ac unit is for my room ?
b. Could someone accidentally switched off the power to the ac unit ?

I have a O'General Duct Thermostat, Model No : ARG30ALC.

Thank You

On 2017-06-07 by (mod) - thermostat G-wire


If the furnace never used the G-wire then I suspect all changes or jumpers ought to be handled right at the thermostat.

Let's review what your thermostat needs to control (heat only?) and to do that let's start with the brand and model thermostat so we can take a look at the installation instructions.

I also need to know just what thermostat wires are

1. present coming out of your wall at the thermostat

2. present and connected to anything at the other end of the same wire set at your heater

On 2017-06-07 by Ann

I got a new Wi-Fi thermostat and in un-installing the old one I did not have a c wire. The instructions stated to put the g wire into the c spot then open the furnace are and switch it there also and create a jumper from the g to the y. In opening the furnace I had wires to both c and g.

In leaving them as is the new thermostat was not coming on. In putting the g wire into the c (And capping the c) it's on and running but not cooling. I did do the jumper as the g and y were not next to each other. Should I put the c in the g spot? Thus essentially switching them? Nothing is in the g spot now at the furnace....

On 2017-05-19 by (mod) - switch on your thermostat won't change when it should


If the switch on your thermostat won't change when it should, either the thermostat is defective or there is a heating or cooling system condition and problem on its other end that we don't understand.

You're citing a Honeywell DT 92E Wireless Room Thermostat with ECO energy saving feature. That thermostat includes an Off/standby button, allowing manual switch off, with frost protection active.

Off/Standby Button, with Frost Protection

The off/standby button allows DT92 to switch off the heating (or cooling) system at the touch of a button.

To prevent accidental switch-off, the button must be held for 2 seconds to activate the change.

When off, DT92 will maintain control at a frost protection setpoint, factory set to 5°C but adjustable between 5 and 16°C.

If required, frost protection can be switched off, to provide a positive off function. These adjustments are made by entering the DT92 Installer Mode.




Both of those are PDF files also available directly from Honeywell

On 2017-05-19 by Liam

My DT92E thermostat seems to be stuck on standby

On 2017-05-19 by (mod) re: how to get the Honeywell DT92E thermostat in or out of Standby mode


If the switch on your thermostat won't change when it should, either the thermostat is defective or there is a heating or cooling system condition and problem on its other end that we don't understand.

You're citing a Honeywell DT 92E Wireless Room Thermostat with ECO energy saving feature. That thermostat includes an Off/standby button, allowing manual switch off, with frost protection active. This instruction from Honeywell should help you -

Off/Standby Button, with Frost Protection

The off/standby button allows DT92 to switch off the heating (or cooling) system at the touch of a button. To prevent accidental switch-off, the button must be held for 2 seconds to activate the change.

When off, DT92 will maintain control at a frost protection setpoint, factory set to 5°C but adjustable between 5 and 16°C. If required, frost protection can be switched off, to provide a positive off function. These adjustments are made by entering the DT92 Installer Mode.

See this HONEYWELL DT 92E THERMOSTAT SPECIFICATIONS sheet that describes the DT92E thermostat features

and be sure to

See this HONEYWELL DT 92E THERMOSTAT INSTALLATION MANUAL where you'll find instructions including how to set the off/standby button and how to enter the DT92 "Installer Mode" when needed.

Both of those are PDF files also available directly from Honeywell

Other readers, see THERMOSTAT WIRING HONEYWELL for a complete list of Honeywell thermostat wiring and operation manuals that can be downloaded as free PDFs

On 2017-05-19 by Liam

My DT92E thermostat seems to be stuck on standby

On 2017-03-20 by (mod)

Readers: if you're stuck with no heat, a place to start looking is at the thermostat - the article THERMOSTAT WON'T TURN ON

But if it's not a thermostat problem, then see HEAT WON'T TURN ON

On 2017-02-20 by (mod) - what about a thermostat that won't turn off: different articles

Duane: the article above on this page is for heat that won't turn ON,

Searching using the search box just above finds two good diagnosis and repair articles for heat that will not shut down:




and if your heat is a furnace system (forced air) and the blower won't turn off,

On 2017-02-20 20:46:13.792753 by Duane

Heater will not shut down

On 2017-01-14 by (mod) re: signs of a broken thermostat wire or loose connection in the thermostat itself


I suspect a broken or disconnected thermostat wire or more rarely, a bad thermostat.

On 2017-01-11 by David Florida

My wall thermostat will not work when I even changed the batteries. The fan will come on or can turn it off; but the A/C or Heat will not come on. The window will power on showing nothing but a light...nothing else. Could it be the Thermostat?

On 2017-01-07 by (mod) re: problem isn't the thermostat but it's at the heater itself


From what you describe I think the problem is not the forced-air blower for your furnace but rather that the heater itself (gas, oil, electric) is not turning on. You may be out of fuel or there may be a problem with the burner or the heater's control. Search InspectApedia for NO HEAT DIAGNOSIS to see details of what to check.

On 2017-01-07 05:13:52.129068 by Tracy

My thermostat brand name is hunter
I turn my thermostat to heat and on auto but it want come on it only come on when I put it on on and it blow cold air out not heat

On 2016-12-24 by con

the heat comes on but not much heat

On 2016-09-22 by (mod) re: Honeywell CM921 1 day wireless programmable room thermostat is not working


First check that power is on to the equipment you're trying to control.


Before the buttons stopped working did you see a Low Battery indicator flashing in the display? The batteries should last about 2 years but may need replacement earlier if they were not fresh to start. This is the MOST LIKELY reason for a blank display.

If the display is not blank but the up and down buttons +/- don't work then the thermostat may be in the wrong mode.

I'd start by resetting the device to factory settings. Follow these instructions:

Press the time button once. The display will show ‘RESET PROGRAM’. Press the or buttons to select the preferred setting (0=user program,
1=factory program) and the display will flash, indicating a change has been made. To confirm this change press the green button.

Next check that the thermostat is in MAN position - there the up and down buttons should work. Here is what Honeywell says:

To operate as a simple thermostat with one temperature for a period of time, move the slider switch to the MAN position. Adjust the manual temperature by pressing the
or buttons. This can be set anywhere from 5°C to 35°C in 0.5°C steps. The thermostat will continue to maintain this temperature until another operating mode is
selected or the temperature is adjusted.

Next: did you install the BR91 relay box? If so check that the green LED is on. On that device the green LED indicates normal operation of the radio frequency system. m. When the green LED is ON the boiler is also ON. When the LED is OFF the boiler is OFF.

If you're controlling AC rather than heat,

Honeywell explains:

if your system has been set up correctly you can use your thermostat to control the cooling. To switch the thermostat between heating and cooling modes press the
and buttons together for 5 seconds in any of the product operating modes (AUTO, MAN or OFF). The text ‘COOLING’ will be displayed for a moment on the
screen to indicate the cooling operation is now active. The factory default program for cooling is specified in the table [in the instructions], but this can be modified in the same way as the default heating program.

Last, you can completely disconnect the thermostat and its remote, then connect the TT wires together at your heater or air conditioner to force the unit on. If that works then we know the problem is with the thermostat or its remote control, not the heater or air conditioner.

If none of these helps I'd give Honeywell support a call before going to the trouble of returning and swapping the unit.

On 2016-09-22 by brian

I have a Honeywell CM921 1 day wireless programmable room thermostat wit LOT technology. neither the timing button ( + or -) nor the set temperature buttons will respond and we cannot turn our heating on. We have hot water though. Can this problem be fixed?

On 2016-06-20 by Anonymous

My compressor outside fan won't run I have air in here but it's not cold

On 2016-06-20 y (mod) re: when to check power to the heat pump or air conditioner

CHeck power to the outside compressor/condenser;

On 2016-06-20 by david in texas

My unit is NOT working not even the oitside unit but the blower inside is blowing is....i change the batteries and still nothing

On 2016-06-17 by (mod) re: air handler runs but no cool air if there is no refrigerant

Shelly The air handler may run (the blower moving indoor air) and the outdoor compressor/condenser may run, but the system will not produce cool air.

On 2016-06-17 by Shelly

Will ac come back on with out gas

On 2016-06-02 by (mod) re: HVAC control failure versus refrigerant leaks

A control or relay can fail "out of the blue"; a refrigerant leak usually is slow and affects the system increasingly over time. Low refrigerant can cause coil icing during long on-cycles; then you no longer get cool air.

On 2016-06-02 by Anonymous

First, thank you for the reply. Would that be something that could start happening out of the blue. It has worked fine for several years, until about two months ago. Sometimes turniing it off and taking the batteries out for 30 minutes fixes it, but not always.

On 2016-06-02 by (mod) re: A/C can't reach the low set temperature on the thermostat: why?

Your question suggests that the AC unit is not able to reach the lower set-temperature; the problem could be heat gain, air leaks, or a problem with the blower (low air flow) or the compressor.

On 2016-06-01 by Anonymous

a/c wont kick off unless set to a higher temp, and then it shuts off normally. If left to run it stays at same temp, and sometimes -after an hour- starts to blow warm. Is that a thermostat control?

On 2016-05-27 05:48:46.811446 by Kenneth Le

The cooling system option won't show, only the heating system and off options will show

On 2016-04-03 by (mod) re: blower won't stop running

Robert: searching for BLOWER FAN WON'T STOP yields this diagnostic article

let me kinow if you need further help.

On 2016-04-03 by Robert

With heat off, fan continues. How to stop fan?

On 2016-03-18 by (mod) re: red flags warn about an HVAC contractor

Thanks Freezing.
Actually I'm still in Freezing Northern Minnesota - happily the heat is working - at least indoors.

On 2016-03-18 by Freezing Cold

B-I-L told us there were a number of red flags with this guy that could have saved us time, money & aggravation from hiring a HV contractor who was "in over his head" on this job..

.like not returning our calls for service, pressuring us, during a cold snap,he could get the furnace working again if we paid to replace all of its internal parts & finally, when he went ballistic at my wife, because he said she asked too many questions.

Glad you made it back from frigid nailed this problem right away....many thanks danjoe!

On 2016-03-17 by (mod)


Thank you for the feedback - it'll help other readers. And apologies for not replying immediately: I was traveling to freezing-cold northern Minnesota when you posted your query and I just managed to thaw out my computer today.

Indeed you're describing one of the two most common problems with thermostat wires

1. wires that are cut, broken, open somewhere in the circuit, including where the wire end is supposed to be in a connecting clamp or under a screw: sometimes this fault can be subtle, such as insufficient insulation stripped from a wire end causing the screw, when turned down, to simply pinch on the insulation rather than contact the bare copper of the wire

The thermostat won't turn ON


2. thermostat wires that are nicked or over-stripped and are shorted together: the thermostat won't turn OFF

On 2016-03-17 by Freezing Cold

danjoe: My brother-in-law, an HV contractor discovered heating problems we had been experiencing, were the result of thermostat wires being stripped & not making contact after "Mr. Nothing's Wrong" squeezed them through a too small opening. Furnace is working properly since damaged sections of wires were removed & right-sized thermostat installed.

On 2016-03-09 by Freezing Cold

Furnace power-pile (thermocouple) was just replaced January 2016. Gas company technician could find nothing wrong with our home's gas pressure. The only other thing left to check out is the wall furnace's electrical lines. Very frustrating situation.....since the furnace pilot light never continued going out with the old manual thermostat. Your advice is appreciated.

On 2016-03-05 by (mod) re: thermostat won't work in AUTO mode

Stephanie it sounds as if you need a service call to repair your heating system. The FAN ON switch is simply turning the fan on manually. That won't turn on heat.

If the thermostat is set well above room temperature and the heat does not turn on, then you can check the RESET button on your oil burner (IF your heat uses oil) and you can press it ONCE. If heat starts and runs you may stay warmer while waiting for repair.

On 2016-03-05 by Stephanie

Our Honeywell thermostat works when the fan is turned to on but doesn't when on auto. The heat and cold also do not work.

On 2016-03-03 by (mod) re: when to replace a thermocouple on a gas burner

What was suggested is plausible but I'd consider replacing the thermocouple as that's easy, cheap, and often the trouble.

Your gas company or a trained service tech can easily measure gas pressures.

On 2016-03-01 by Freezing Cold

Wall furnace pilot went out again during the night & we woke up to no heat. Service tech checked out furnace & thermostat, then got pilot to re-light right away. He thinks either the furnace has a gas pressure problem, which needs to be determined by our gas company, or it's a problem with the furnace's electrical line, which he says is an easy fix. Do you agree? Thanks

On 2016-02-14 by (mod) re: how to rule out the thermostat as the problem cause in a no-heat or no-cooling issue


You can determine if the problem is the thermostat or the thermostat wiring by eliminating the thermostat entirely from the equation. If you remove the thermostat and then simply connect the two heat wires of the thermostat circuit together you're doing exactly what the thermostat itself would do on a call for heat - it's basically just an on-off switch.

So without the TT in place if you connect the wires the heat should run continuously.

On 2016-02-14 by (mod) re: if the batteries are dead will the thermostat still run the furnace?


I've had this trouble when the springs or contacts in the battery compartment were damaged, bent, or not making full contact. Check that all of the contacts are clean - maybe even wipe them off with an alcohol-dampened cotton swab. Then slightly stretch out the spring lengths so that the spring pushes the battery firmly in place. Then check that the flat contact at the other battery end is making good contact.

With dead batteries, most but not all thermostats will still work; in those thermostats the batteries are there to keep from losing the thermostat's program during a power outage. But there are some thermostats that will not work without batteries - typically because their power draw is more than that provided by the thermostat wires. The instructions for your thermostat should indicate which type you have.

On 2016-02-14 by Skylar Rote

Thermostat said change batteries. We did. It still said change batteries. We found out if we do not close the batteries top all the way it works fine. When we close the top it once again says change batteries. We have to sit and hold it half way closed in place in order to make it work.
If the batteries are dead will the furnace still run if it a wired thermostat?

On 2016-02-14 by freezing cold

None of the pins inside thermostat wall plate look loose.....anything else I can try before buying another thermostat or calling for service again? This never happened with original non-digital furnace thermostat. Neighbor suggests old/faulty furnace wiring might be causing this problem. Thanks again.

On 2016-02-13 by (mod) re: signs of a bad thermostat wire connection

That suggests to me that there's a bad wire connection, Freeze. Digital thermostats don't care about being out of level as the temperature sensor is a tiny thermosensor. (See details about thermistors at )

"Nothing wrong" is baloney if the thermostat doesn't call for heat. A more honest answer would have been "I don't know how to diagnose this and I give up"

Most digital thermostats connect a bunch of pins on the back of the thermostat into a terminal block in the wall plate. If a wire is loose at the wall plate or if the thermostat itself is defective - for example if one of its pins is loose - your ajar cover explanation may be diagnostic.

It's easy to swap in batteries - worth doing, but I don't know why that'd work or not depending on cover position.

Start by confirming that the heat works properly if you simply jump the two thermostat wires at the furnace or boiler end of the circuit. After all a thermostat is basically a simple low-voltage on-off switch.

On 2016-02-13 by freezing cold

Our new digital thermostat works fine, as long as its cover is slightly ajar, but when that cover is set flush against the wall, the furnace won't heat. Had it checked out by a local HV service person who says nothing's wrong with it. Don't know if it's not level or due to bad senors, wiring, batteries.....any ideas/suggestions greatly appreciated.

On 2016-02-06 by (mod) re: how big a part does the thermostat play in keeping the heating furnace running?


I would stop playing around with the thermostat. Rapid turning on and off of the furnace can cause it to have worse problems, particularly with oil fired heating equipment.

The problem may be at your furnace, or there could be a loose thermostat wire. I suggest calling the repair company.

On 2016-02-06 by Beth

Just unsure as to how big a part thermostat plays starting and keeping your furnace running

Know have thermostat issues, but when play around with it furnace will kick on, but then dies out. Will attempt to start about 3 times. Can this still be thermostat issue? Or once it starts and dies, it is probably furnace issue?

On 2016-01-24 by (mod) re: why the cooling or heating system seems inadequate


If your building's rate of heat loss to the outdoors exceeds the capacity of your heating system that could be the problem; look for drafts, air leaks, heat leaks;

If your heat is by forced warm air, search InspectApedia for WEAK FURNACE AIRT FLOW for details.

On 2016-01-20 by Fred

The heat will not go past 18c on the thermostat. Why does it not go higher.

On 2015-12-21 y (mod) re: fuse keeps blowing when I turn on the thermostat


You don't say which fuse but I suspect that you mean the fuse that powers the 24VAC tranformer that in turn powers your thermostat. If so, I suspect that either the thermostat wires are shorted together or the transformer has failed.

On 2015-12-21 by grady western

Fuse keeps blowing when I turn on thermostat, what is causing it?

On 2015-12-21 by (mod) re: heat upstairs but no heat downstairs


Do you have two different furnaces for upstairs and downstairs? If so check the downstairs unit: it sounds as if your heat source, such as an oil burner, is not working.

On 2015-12-20 by Gerrard

Thermostat comes on upstairs and the heat is running good, but downstairs it comes on but no heat is coming out of the vent, only cold air.

On 2015-12-01 by (mod) re: why won't my thermostat hold temperature?


IF the problem is the thermostat itself that's easy to test: your heating service tech can simply jump the thermostat wires at the heater: if it runs and keeps running the issue is the thermostat or its transformer or power supply or wiring.

On 2015-12-01 by veronica

My thermastat wont hold temp. I need to turn off and the. On. Th heat will turn on for about. 5 min then Turn off

On 2015-06-18 by Mrs.Rivera

My central heat and a/c are turn off,the thermostat is on off , but when the temp reaches 80 the heat will start and its on off, what I do is turn the switfh off which turns part of my lights off and I am 72 and can see whats wrong? please help

Question: we replaced the thermostat and now our living area is too hot - and the heater won't turn off

Our living area has been really hot and the old thermostat did not have an ON/OFF switch. We changed the thermostat today and switched it off but the heater is still on. We cannot find a reason why and we have verified the wiring and all is correct. Is it possible that something else may not be working? Maybe the transformer or some kind of regulator?


First disconnect the thermostat wires right at the heater itself, at it's control. That's the same as turning down the thermostat. If the heater stops then you know the trouble is in the thermostat wiring thermostat settings, is the thermostat itself.

As your problem showed up after installing a new thermostat, I would be sure to check that you made the right connections, that the thermostat switches are properly set, AND that the thermostat wires were not accidentally shorted together.

Question: thermostat - zone valve control troubleshooting

(Dec 4, 2014) Dominick said:
Having an issue with my thermostat/zone valve. I was getting heat to a zone when that zone was off. I replaced the 24v motor to that zone valve because after troubleshooting I realized the valve would not open all the way when asked and it would stay open slightly, causing the hot water heat to flow to that zone when the other 2 zones were on. After installing the new motor and setting the temperature for that zone 5 degrees above the actual room temperature, the valve opened 100%.

I was happy to see that the valve now opens and closes properly. The old motor did not offer enough tourque to open fully. Well the issue now is the circulator pump is not turning on for this zone. So what do you think it is? I checked the 2 wires connected to the thermostat and the wires/screws are tight.


Nice work Dominick

Check that the end switch wires on the zone valve are calling for heat when the valve is fully opened.

Question: wood furnace fan control wiring & buzzing

(Feb 12, 2015) Jon said:
The fan center on the back of my indoor wood furnace (where the thermostat wires come in and the main power) was buzzing tonight while it was calling for heat. I checked all the wires, moved them all just a little, made sure the connections were good, and then it stopped.

Does that sound like a loose connection with the wires, or does it sound like the fan center could be going bad?



I am not sure from just the info in your note; but buzzing can indeed indicate an unsafe, overheating, burning electrical connection;


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