Unit does not start
	Insufficient cooling
	Over cooling
	High head pressure
	Low head pressure
	High suction pressure
	Low Suction Pressure
	Ice or frosted Suction line
	Compressor Short cycles
	Compressor does not start
	Compressor runs non stop

Unit does not start   Check thermostat  Turn fan to the on position if the furnace fan starts go check for power at the condensing unit. Check disconnect , breaker or fuses. Lets not forget about that switch at the bottom of the stairs.  Check Contactor If the contactor is pulled in, then the problem is most likely a line voltage problem. If the contactor is not pulled in the problem is in the low voltage side of the unit. Check to see if the coil is energized. Check low ambient stat.  If open you can temporarily bypass it. Check time delay relay If open you can temporarily bypass it. Is the low pressure control open ? See low suction pressure. DO NOT BYPASS  Is the high pressure control open ? See high head pressure. DO NOT BYPASS

Insufficient Cooling

Check refrigerant charge	Either an undercharge or over charge can cause this.
Is the condenser restricted ?	Look for leaves, paper ect.
Is the condenser clean ?	Clean it out with a garden hose. TURN OFF POWER FIRST. to clean it spray the water through the fins in the condenser.
Is there frost on the suction line ?	This is usually a sign of either loss of refrigerant charge or most commonly lack of air flow. See the next questions.
Are the filters plugged ?	result will be an  extra cold suction line and low back pressure.
Is the evaporator clean ?	result will be an  extra cold suction line and low back pressure.
Do you have sufficient air flow ?	result will be an  extra cold suction line and low back pressure.
How many air diffusers are closed ?	Too many closed will result with extra cold suction line and low back pressure.
What is your Supply air temperature ?	Look for a TD of 15 to 20oF
Is the compressors drawing RLA ( rated load amps) or less ?	If it's less are valves ok ? look for low head and higher than normal back pressure.

 

Over cooling

Check thermostat	Is it calibrated, sticking on or is it in an improper location.
Check the compressor contactor	Are the contacts welded closed.

High head pressure

Check refrigerant charge	Overcharge will reduce the capacity of the condenser
Is the condenser restricted ?	Look for leaves, paper ect.
Is the condenser clean ?	Clean it out with a garden hose. TURN OFF POWER FIRST. to clean it spray the water through the fins in the condenser.
Does the fan motor seam to be running properly?	.
Does the system have non condensable in it?	Sometimes noted by excessive subcooling or at least what appears to be according to a PT chart.
Is you load extremely high	Higher than normal back pressure. if so cool condenser with water to catch up with the load then verify unit sizing and charge.
Is condenser air re circulating	unit under a deck or low roof

Low head pressure

Check refrigerant charge	Lack of refrigerant . DO NOT USE REFRIGERANT TO TRY TO INCREASE PRESSURE.
Out door air to low for proper operation	Normal units should not be run below 55oF without winter operation kits installed.
No load on the evaporator	Dirty filters, closed vents, dirty coils , incorrect fan speed , all will be noted with low back and possibly frost.
Compressor valve damage	Diagnosed by low head, high back and low current draw.

High suction pressure

Check Charge	Overcharged
Compressor Valve damage	look for low head pressure and low current draw.
high head pressure	resulting in increased pressure drop on metering device, refer to High head pressure.
Excessive Load	Unit too small
Furnace heat on	.

Low Suction Pressure

 

Check refrigerant charge	Either an undercharge or over charge can cause this.
Is there frost on the suction line ?	This is usually a sign of either loss of refrigerant charge or most commonly lack of air flow. See the next questions.
Are the filters plugged ?	result will be an  extra cold suction line
Is the evaporator clean ?	result will be an  extra cold suction line
Do you have sufficient air flow ?	result will be an  extra cold suction line
How many air diffusers are closed ?	Too many closed will result with extra cold suction line
What is your Supply air temperature ?	Look for a TD of 15 to 20oF
Is the outdoor air temperature too low for proper operation	Install head pressure controls if you require low ambient operation

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Ice or frosted Suction line

 

Check refrigerant charge	Either an undercharge or over charge can cause this.
Are the filters plugged ?	.
Is the evaporator clean ?	.
Do you have sufficient air flow ?	.
How many air diffusers are closed ?	Too many closed will result with extra cold suction line
Is the fan belt broke ?	Look for a TD of 15 to 20oF
Is the outdoor air temperature too low for proper operation	Install head pressure controls if you require low ambient operation

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Compressor Short cycles

 

Thermostat Incorrectly installed	Supply air is influencing it. or is there a door that shakes it when the door closes.
Is it cycling on overload	Is the compressor overheating ? , Sufficient voltage at the unit ?
Check charge.	Is it cycling on low or high pressure See low suction or high head
Is the unit oversized.	check square footage of home

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Compressor does not start

Check thermostat	Turn fan to the on position if the furnace fan starts go check for power at the condensing unit.
Check disconnect , breaker or fuses.	Replace as required
Check Contactor	If the contactor is pulled in the problem is a line voltage problem. If the contactor is not pulled in the problem is in the low voltage side of the unit. Check to see if the coil is energized.
Check low ambient stat.	If open you can temporarily bypass it.
Check time delay relay	If open you can temporarily bypass it.
Is the low pressure control open ?	See low suction pressure. DO NOT BYPASS
Is the high pressure control open ?	See high head pressure. DO NOT BYPASS
Is the compressor hot ?	Off on overload
Start or run cap shorted, open or blown.	If unit humming check these,
Compressor hums	Start or run cap shorted, open or blown, If seized try a hard start kit. do not call compressor if it is hot. Only call a cold compressor

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Compressor runs non stop

Check refrigerant charge	Either an undercharge or over charge can cause this.
Is the condenser restricted ?	Look for leaves, paper ect.
Is the condenser clean ?	Clean it out with a garden hose. TURN OFF POWER FIRST. to clean it spray the water through the fins in the condenser.
Is there frost on the suction line ?	This is usually a sign of either loss of refrigerant charge or most commonly lack of air flow. See the next questions.
Are the filters plugged ?	result will be an  extra cold suction line and low back pressure.
Is the evaporator clean ?	result will be an  extra cold suction line and low back pressure.
Do you have sufficient air flow ?	result will be an  extra cold suction line and low back pressure.
How many air diffusers are closed ?	Too many closed will result with extra cold suction line and low back pressure.
What is your Supply air temperature ?	Look for a TD of 15 to 20oF
Are the low and high side the same pressure	Cycle unit and to verify that the high pressure internal relief valve is not causing the problem prior to calling the compressor.
Is the compressors drawing RLA ( rated load amps) or less ?	If it's less are valves ok ? look for low head and higher than normal back pressure.

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