Diagnose an Air Conditioner / Heat Pump that Won't Start

10 Easy Checks Fix Most A/C or Heat Pump No-Start Problems

Step 1: Check the Room Thermostat Temperature Setting

Has the cooling thermostat temperature been set below ambient room temperature? Our Honeywell digital thermostat above shows that the room temperature is 67° F.

Set the room thermostat to at least 5 degrees below room temperature.

My elderly mom had no patience with switches and controls.

She regularly called her air conditioning service company with a service request, sometimes late at night, because she simply failed to set the temperature on the thermostat lower than the room temperature.

Don't drive your A/C service technician mad like my mother did.

Step 2: Check that the Room Thermostat is set to "Cool" 

Be sure the thermostat mode switches are not set to "Off" or "Heat". Our photo below shows that the thermostat is set to HEAT mode - not COOL, so the air conditioner won't run.

If the thermostat is not set to "cool", it is simply turning off your A/C.

If the thermostat display is blank, then it's not receiving power (for modern digital thermostats). Check that electrical power is on at the air handler and to the the low-voltage transformer that supplies power to the thermostat.

If the thermostat has power, then check for broken or shorted thermostat wires anywhere between the wall thermostat and the control board at the air handler.

Tip: You can easily eliminate possible thermostat problems as a cause of failure of the air conditioner to start by simply eliminating the thermostat from the picture: disconnect the thermostat wires at the blower unit's control board and instead connect the two thermostat terminals directly together with a jumper wire. If the system starts then the problem is in the thermostat itself or in its wiring.

See THERMOSTATS for a details of how air conditioner thermostats work and how the air conditioner blower fan controls work.

See THERMOSTAT SWITCHES, EXTERNAL  if you need help understanding all of the controls and switches on your room thermostat.

See THERMOSTAT DIAGNOSTIC GUIDE if your thermostat is not working

Step 3: Check that Electricity is Turned ON for the A/C Equipment indoors & outside

Do both the outdoor compressor/condenser unit and the indoor blower fan/evaporator coil unit have electrical power?

Has the air conditioning electrical wiring been physically damaged or cut? Our photo above shows damaged electrical wiring at the outdoor compressor/condenser unit, courtesy of Tim Hemm.

Are the power switches ON at these units, are the fuses good, are the circuit breakers in the "on" position, and is the thermostat set correctly?

Check all of the electrical switches and controls that can turn electrical power off at the indoor air handler or at the outdoor compressor/condenser unit.

There are more of these switches than you might guess. Here's a list of what to check:

Main Electrical Power Switches for an Air Conditioner or Heat Pump

You will find service switches outside by the compressor, inside at the air handler, and you will also want to check the fuses or circuit breakers in the electrical panel.

• Check for a blown fuse locally
  
  inside the air handler, such as on or near a control board if switches are all on but there is no power being delivered to the indoor blower fan or outdoor compressor system.
  
  

Step 4: Check the electrical supply voltage

Even if electricity is turned on, if the supply voltage has fallen too far below the operating voltage range of your air conditioner it's likely that the system will not operate, particularly, you may note that the compressor motor won't start.

See VOLTS MEASUREMENT METHODS

Watch out: if you hear a humming noise at an electrical motor that won't start, turn the system off while waiting for repair. Shutting off the system might avoid costly damage to the equipment.

Watch out: if you are trying to run your air conditioner or heat pump from a backup electrical generator, it may be that while the generator supplies enough current (Amps) to run the equipment, it is not capable of handling the extra current draw (high amps) required to start the motor.

You may be able to circumvent this problem with an extra-boost or "hard-start" motor capacitor.

See CAPACITORS for HARD STARTING MOTORS

Step 5: Check the Indoor Air Handler / Blower Unit: Does it Start?

At the air handler there are several switches and controls, both manual and automatic, that can leave an air conditioner or heat pump turned "off" such as a blower compartment door interlock safety switch.

• BLOWER DOOR SAFETY SWITCH - an open blower door will shut down the air handler unit
• MOTOR OVERLOAD RESET SWITCH - an overheated indoor blower fan motor could be keeping the blower unit fan from starting. If your blower motor will run after it has been shut down for a few hours (to cool off), we suspect this is the trouble.
  

  And if your blower fan is driven by a fan belt and an electric motor, of course check to see that the drive belt is in place and undamaged. If the blower fan belt is broken, the electric motor may run just fine (you may be able to hear it) but the blower fan assembly itself won't be turning.

  Details are at BLOWER FAN OPERATION & TESTING

  Similarly, the outdoor compressor/condenser unit also has its own cooling fan that could be off due to a problem with its fan motor, as does the compressor/condenser motor itself.
  
• CONDENSATE PAN SWITCH LOCKOUT - a condensate tray spillage detector switch (water in the overflow tray shuts down the air handler) will shut down the blower fan unit.
• Check (or your service tech will check) for a bad contactor or start relay for the Air Handler
  
  inside the air handler or outside at the compressor/condenser unit . These electro-mechanical switches are turned on by the unit's control circuit board and in turn they cause the blower motor or compressor motor (outdoors) to turn on or off as needed.
  
  More diagnostics for a contactor relay that won't pull in or close are
  
  at START RELAY DOESN'T CLOSE
• Check (or your service tech will check) for a bad or failed starter capacitor
  
  for the fan motor in the blower assembly or outdoors at the compressor/condenser unit. This could also be leaving your system shut down, failing to start a blower motor fan indoors or outside the compressor condenser unit's fan, or compressor motor.

  If an electric motor hums but won't start or won't keep running, the problem could be a bad start/run capacitor.
  
  See CAPACITORS for HARD STARTING MOTORS
  
  Of course a frozen bearing or burned-up compressor or motor or even low voltage can also mean a motor hums but won't start .

• Watch out: See A/C - HEAT PUMP CONTROLS & SWITCHES to be sure you have found and checked every manual or automatic electrical switch on the system.

Step 6: Air Conditioner Won't Start or Stays Off Due to Condensate Pan Switch or Door Switch

Shown above is a FLOAT SWITCH on Condensate Tray that could cause a "condensate pan switch lockout" that prevents the blower unit from running.

Some condensate pans have their own separate overflow drain (a proper installation) or share their drain with the normal condensate drain (an improper installation).

This drip tray or drip pan, located under an air handler, is installed to catch air handler condensate leaks if the normal condensate drain system fails.

But on some air conditioning air handlers, the installer may provide a condensate overflow pan switch rather than a separate pan drain line.

Condensate spilling into an overflow pan that uses a sensor switch can be enough to shut down your air conditioner.

In this installation the switch is designed to turn off the air conditioning system if it finds condensate water in the overflow pan. The idea is to shut down the air conditioner before there is a more costly leak into the building insulation or ceiling.

Condensate Drain Failure

can also shut down the air conditioner.

In tracking down an air conditioner condensate leak, I found that the the condensate pump drain line, a small-diameter PVC pipe, was clogged with water mold.

That was why my air conditioner wasn't kicking on. It would of ended up costing at least a hundred dollars just for a service tech to fix something as simple as that. I had to take the pump apart and clean it. I took off the PVC drain line coming out of inside air conditioner and blow and clean it all out too. - Jacob Behrends, FL

The central a/c in my Florida home (in late July) would not turn on and the temp in the house was well above the preset temp of 78. Fearing a complete failure (the system is 17 years old) I googled my problem and eventually found this very clear and concise Inspectapedia site.

The past few days I could hear water dripping in the condensate drip pan and new something was up as this never happened before. I climbed my ladder at 11:30p and sure enough the condensate float switch was parallel to the tray meaning it tripped and turned the a/c system off.

Grabbed my shopvac and within minutes the immediate problem was solved - Now to clear the drain...Excellent DIY website! (July 24, 2014) Randy

So if your air conditioning system seems to be normal in all other respects but it simply won't turn on, check for a flooded condensate pan or a defective condensate switch.

See CONDENSATE PAN OVERFLOW SWITCH in our description of HVAC switches & controls

See CONDENSATE DRIP TRAY DEFECTS

Also check for an open Blower Compartment Door Safety Interlock Switch

Watch out: if someone has opened or removed an access door or cover to the air handler it's possible that a blower door safety switch is keeping the unit from turning on.

To protect people from getting a finger chopped off by the spinning blower fan, a door safety interlock switch will open (turning off power to the blower) if the blower compartment door is not in place and properly latched.

For more help

see BLOWER DOOR SAFETY SWITCH 

Step 7: Check the Outdoor Compressor / Condenser Unit: Does it Start?

Relays on the control board for the compressor/condenser unit start the compressor motor and the condenser unit's cooling fan individually, but they may be related as we'll explain. Check the outdoor components in this order:

1. Is there power to the compressor/condenser unit? 

Check that the main circuit breaker or fuse is on, that the outdoor service switch is on. If the compressor/condenser circuit's main breaker is tripped or fuse blown the system may have a seized compressor motor or a shorted electrical wire.

Back at AIR CONDITIONER WON'T START we reminded you to check that the thermostat was calling for cooling. If you forgot to do that, go back and check that article.

2. Is the refrigerant pressure in the system at the pressure level equivalent for the ambient temperature at the condenser unit?

Modern compressor units include both high and low refrigerant pressure safety cut-out switches that can shut down the equipment in abnormal conditions.

Watch out: a refrigerant leak and low refrigerant can leave a compressor motor running so long that the motor overheats. An overheated A/C or heat pump motor may shut down on thermal overload, or it may simply burn-up and thus be destroyed.

3. Is the line voltage delivered to the compressor/condenser unit normal? 

Occasionally low voltage will prevent equipment from operating or will shut it down. Low voltage can also mean motors have trouble starting or run at abnormal power levels.

4. Is electrical power delivered to the compressor & to the condenser fan? 

Look for burned, shorted, disconnected wires supplying these two motors.

5. Is voltage being delivered to the start relay - the compressor contactor?

If you see control voltage (typically 24VAC) at the contactor and you have line voltage to the compressor/condenser unit but the relay does not pull in, then probably the contactor is bad (or there is a blown fuse on the control circuit board). Next we give diagnostic steps for the following situations

1. COMPRESSOR START RELAY DOESN'T CLOSE
2. COMPRESSOR START RELAY CLOSES, COMPRESSOR DOESN'T RUN
3. CONDENSER FAN DOESN'T RUN

Step 8: HVAC Contactor or Start Relay Won't Close

If the A/C or heat pump compressor motor start relay does not pull in (close) then it is not asking the compressor to run. Check the following:

If there is no control voltage at the compressor's start relay and there is electrical power to the unit and the thermostat is calling for cooling, then one of the safety cut-out switches we mentioned above may have tripped.

Check each of those safety relays for voltage on the relay output side. If there is no voltage then that relay has tripped (or failed).

If you hear a buzzing or rapid clicking at the condenser unit (or at some indoor air handlers) the problem may be a chattering relay that is not pulling-in to turn on the unit.

First find and check the following:

Low pressure safety switch:

If abnormally high pressures (typically over 55 psi) are found on the low-side of the compressor/condenser unit, this switch will open to stop the system.

The low pressure switch normally will reset automatically when the low side pressure drops. If it does not re-set this switch has probably failed.

Condenser oil level safety switch:

Some compressor/condenser units (particularly larger and commercial systems) include a safety switch that monitors the oil level in the compressor motor.

Low compressor motor oil levels can shut down the system.

Watch out: most residential cooling systems use a hermetically sealed compressor motor; oil is not normally added to these systems.

If you add oil to a commercial compressor/condenser system you need to check oil levels again after the system has returned to operation to be sure that oil levels are not too high - a condition caused when oil previously left in the piping system has returned to the compressor.

High pressure safety switch:

The compressor/condenser over-pressure or "high pressure" relay safety switch.

A failed condenser unit fan motor or a damaged fan itself (motor spins but fan does not) can lead to compressor/condenser unit high temperatures and pressures.

If refrigerant pressures in the system reach unsafe or abnormally high levels this relay will shut down the compressor motor. A clogged condenser or a failed thermostatic expansion valve can also cause abnormally high pressures in the system.

We first mentioned

at NOISES, COMPRESSOR CONDENSER that we had a field report from a reader who explained that a noisy outdoor compressor unit was, according to his HVAC service technician, traced to a blocked, clogged outdoor condensing coil. We speculate that perhaps the compressor was running hot and that correcting air flow across the condensing coil corrected that condition.

Compressor motor protection switch:

Some compressor systems include a protection module at the main power terminals for the compressor motor. This switch may have opened.

Voltage safety switch:

The voltage monitoring or phase protector relay (not present on all equipment): transient low voltage can trip this relay. It may require manual reset.

Watch out: some electrical motor safety switches require a manual reset. For examples see this separate article on

the MOTOR OVERLOAD RESET SWITCH

Chattering relays, cause and diagnosis are explained in detail

at RELAY CONTROL SWITCHES

Step 9: Start Relay Closes but Motor Won't Run

If the compressor motor start relay pulls in (closes) but the compressor does not run:

Check for voltage at the compressor motor itself.

If there is no line voltage, check the main breaker or fuse or the local service switch again.

Is the compressor motor humming?

If you have a hard-starting air conditioner compressor that "hums" but doesn't start, it may be possible to get more life out of the compressor motor with a simple starting capacitor (rather than a costly whole new compressor motor).

See

• HARD STARTING COMPRESSOR MOTORS
• CAPACITORS for HARD STARTING MOTORS
• TEST a MOTOR START or RUN CAPACITOR

Step 10: HVAC Condenser Unit Fan Not Working

If the compressor motor will run but the condensing unit's fan does not run:

Check that the fan blades will spin - with power off.

If the fan motor won't turn, it's seized and needs to be replaced.

Check the fan relay switch:

If the fan relay does not pull in and there is control voltage to the relay, then the relay is probably bad. If there is no control voltage, check the low voltage transformer and circuit wiring.

Note: some systems include a low ambient temperature control relay that will keep the system from running at low temperatures.

If the fan relay does pull in but the fan does not run, check the line voltage and wiring to be sure power is being delivered to the fan.

Check for voltage at the fan motor: 

and go back to check circuit breaker or fuse if there is none.

If spinning the fan will start the motor: 

and it keeps running, then the motor's start capacitor is probably bad.

If the fan motor has voltage but will not run at all then:

the motor may have failed or the motor's run capacitor may have failed.

See

• LOCATE the STARTING CAPACITOR
• TEST a MOTOR START or RUN CAPACITOR
• ELECTRIC MOTOR DIAGNOSTIC GUIDE

Check (or your service tech will check) for a bad contactor or start relay

inside the air handler or outside at the compressor/condenser unit . These electro-mechanical switches are turned on by the unit's control circuit board and in turn they cause the blower motor or compressor motor (outdoors) to turn on or off as needed.

Check (or your service tech will check) for a bad or failed starter capacitor

for the fan motor in the blower assembly or outdoors at the compressor/condenser unit could also be leaving your system shut down, failing to start a blower motor fan indoors or outside the compressor condenser unit's fan, or compressor motor.

If an electric motor hums but won't start or won't keep running, the problem could be a bad start/run capacitor.

See CAPACITORS for HARD STARTING MOTORS

Of course a frozen bearing or burned-up compressor or motor or even low voltage can also mean a motor hums but won't start .