Air conditioner troubleshooting guides

54 guides. Not cooling, freezing, short cycling and noise on air conditioners. Each one lists the likely causes and what to check, in order.

All brands (41)

• Bubbles in sight glass / temperature drop across liquid line drier
  Possible causes and checks for: bubbles in sight glass / temperature drop across liquid line drier.
• Coil iced, airflow blocked, poor cooling, high box temp
  Possible causes and checks for: coil iced, airflow blocked, poor cooling, high box temp.
• Continuous run, no cooling, condenser cooler than normal
  Possible causes and checks for: continuous run, no cooling, condenser cooler than normal.
• Evaporator freezing
  Possible causes and checks for: evaporator freezing.
• Freezing in cold weather
  Possible causes and checks for: freezing in cold weather.
• General service-call symptom list
  Possible causes and checks for: general service-call symptom list.
• Hard starting / stalls on restart
  Possible causes and checks for: hard starting / stalls on restart.
• Humid indoor air
  Possible causes and checks for: humid indoor air.
• Insufficient cooling, frost at outlet of liquid-line filter-drier, both pressures and amps dropping
  Possible causes and checks for: insufficient cooling, frost at outlet of liquid-line filter-drier, both pressures and amps dropping.
• Lack of cooling, very high head, high amps, high suction
  Possible causes and checks for: lack of cooling, very high head, high amps, high suction.
• Liquid floods back to compressor on start / liquid refrigerant in cold condenser
  Possible causes and checks for: liquid floods back to compressor on start / liquid refrigerant in cold condenser.
• Low head pressure persists in winter, or loss of capacity in summer
  Possible causes and checks for: low head pressure persists in winter, or loss of capacity in summer.
• Low head pressure, starved evaporator, high superheat and high subcooling readings, short cycling, poor cooling in winter
  Possible causes and checks for: low head pressure, starved evaporator, high superheat and high subcooling readings, short cycling, poor cooling in win
• Low head, low suction, poor cooling together
  Possible causes and checks for: low head, low suction, poor cooling together.
• Low suction + high superheat (starved evaporator)
  Possible causes and checks for: low suction + high superheat (starved evaporator).
• Measurable pressure drop across suction filter-drier; oil return problems, low suction, loss of capacity
  Possible causes and checks for: measurable pressure drop across suction filter-drier; oil return problems, low suction, loss of capacity.
• Needs refrigerant (symptoms)
  Possible causes and checks for: needs refrigerant (symptoms).
• No cooling
  Possible causes and checks for: no cooling.
• No cooling via stat
  Possible causes and checks for: no cooling via stat.
• No cooling, compressor and condenser fan both dead, supply voltage present at disconnect
  Possible causes and checks for: no cooling, compressor and condenser fan both dead, supply voltage present at disconnect.
• No cooling, compressor runs, condenser fan off, very high head, compressor cycling on overload
  Possible causes and checks for: no cooling, compressor runs, condenser fan off, very high head, compressor cycling on overload.
• No cooling, system running, house 81°F at 72°F set, high temp split, low suction, ice on coil
  Possible causes and checks for: no cooling, system running, house 81°f at 72°f set, high temp split, low suction, ice on coil.
• Noisy
  Possible causes and checks for: noisy.
• Not cooling
  The system runs but the house does not cool, or the outdoor unit does not start.
• Outdoor unit won't start, 24 V present at contactor coil
  Possible causes and checks for: outdoor unit won't start, 24 v present at contactor coil.
• Overcharged system
  Possible causes and checks for: overcharged system.
• Poor cooling
  Possible causes and checks for: poor cooling.
• Poor cooling with liquid level normal
  Possible causes and checks for: poor cooling with liquid level normal.
• Poor cooling, low suction, high superheat, warm evaporator
  Possible causes and checks for: poor cooling, low suction, high superheat, warm evaporator.
• Poor performance
  Possible causes and checks for: poor performance.
• Runs briefly then shuts down, very high head
  Possible causes and checks for: runs briefly then shuts down, very high head.
• Rust on accumulator / vibration absorber leaks
  Possible causes and checks for: rust on accumulator / vibration absorber leaks.
• Starved evaporator (partial frost, poor cooling)
  Possible causes and checks for: starved evaporator (partial frost, poor cooling).
• Superheat above target
  Possible causes and checks for: superheat above target.
• Superheat below target
  Possible causes and checks for: superheat below target.
• System short cycles
  Possible causes and checks for: system short cycles.
• Thermostat suspected
  Possible causes and checks for: thermostat suspected.
• Undercharged system: compressor runs continuously, poor cooling, high power use, warm liquid line and drier
  Possible causes and checks for: undercharged system: compressor runs continuously, poor cooling, high power use, warm liquid line and drier.
• Water dripping inside
  Possible causes and checks for: water dripping inside.
• Won't engage
  Possible causes and checks for: won't engage.
• Won't start / no cooling
  Possible causes and checks for: won't start / no cooling.

Carrier (7)

• Amber STATUS or green COMM light not on
  Diagnosing the outdoor AOC board from its two lights.
• Compressor will not run - winding and electrical checks - Carrier split systems
  Compressor does not start, hums, or trips on internal protection.
• Delay before the compressor restarts
  The outdoor unit waits before starting again - usually normal.
• Hissing from the outdoor unit after it shuts off
  On the Infinity 19VS (24VNA9) a hiss is heard at the outdoor unit for a short time after the compressor stops. This is normally the pressure equalizer
• Outdoor unit unresponsive after several faults
  After some combinations of earlier fault codes the outdoor unit can stop responding even though nothing is broken.
• Superheat too high or too low on TXV system - Carrier split systems
  Charge looks right by subcooling but superheat at the outdoor unit is off; decide if the TXV or the system is the problem.
• Two-stage unit not changing stages - Carrier 24ANB/25HNB/25HCB6
  Unit runs but capacity does not seem to change between low and high stage.

Goodman (1)

• No cooling - R-32 GLXS/ALXS condenser
  Thermostat calls for cooling but the outdoor unit or indoor blower is not doing its part.

KeepRite (1)

• Checking OAT and OCT thermistors - KeepRite C4A3/CXA6
  How to check the outdoor air and coil thermistors behind codes 53, 55 and 56.

Lennox (4)

• Low cooling capacity - check airflow and charge - Lennox EL16KC1
  Unit runs but does not cool well; airflow and charge must both be checked and adjusted together.
• Outdoor unit will not start - Lennox EL16KC1
  Thermostat calls for cooling but the compressor and outdoor fan do not run.
• RDS leak-detected mode and start-up test - Lennox EL16KC1
  The refrigerant detection system (RDS) blower control board shuts down the compressor (and gas burners) and runs the blower when it detects R-454B; it
• Two horizontal lines on display (soft disable) - Lennox communicating
  The control shows two horizontal lines on its 7-segment display (damper module or EIM: green LED 3 s on / 1 s off). The thermostat found a device it d

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.