HVAC Error Library

Category: Carrier Errors

  • Carrier ComfortLink Alarm 12 – Low Suction Pressure Fault

    What Does Alarm 12 Indicate?

    Alarm 12 typically signals a low suction pressure condition in the refrigerant circuit.

    To protect the compressor, the controller may shut down cooling.

    Common Causes

    • Low refrigerant charge
    • Refrigerant leak
    • Dirty evaporator coil
    • Blocked airflow
    • Failed low-pressure switch

    Troubleshooting

    1. Inspect filters and evaporator coil
    2. Measure refrigerant pressures
    3. Check for visible oil residue (leak indicator)
    4. Test low-pressure switch continuity

    Correct the root cause before resetting the alarm.

    FAQ

    Will adding refrigerant fix it?
    Only if the system is properly diagnosed and leaks are repaired.

  • Carrier ComfortLink Alarm 78 – Communication Error Explained

    What Does Alarm 78 Indicate?

    Carrier ComfortLink Alarm 78 generally indicates a communication failure between the main controller and a module or sensor.

    Affected modules may appear offline.

    Common Causes

    • Broken communication wiring
    • Loose terminal connections
    • Incorrect addressing
    • Failed communication module
    • Electrical interference
    • Grounding issues

    Troubleshooting Steps

    1. Inspect Communication Bus

    Check polarity, continuity, and termination.

    2. Verify Addressing

    Ensure no duplicate device addresses.

    3. Power Cycle Modules

    Clear temporary faults.

    4. Replace Faulted Module if Necessary

    Only after verifying wiring and addressing.

    FAQ

    Is this the same as a sensor fault?
    No. It specifically refers to communication failure.

  • Carrier ComfortLink Alarm 53 – Compressor Protection Fault

    What Does Alarm 53 Mean?

    Carrier ComfortLink Alarm 53 typically indicates a compressor protection event triggered by abnormal operating conditions.

    The controller has disabled compressor operation to prevent damage.

    Common Causes

    • High head pressure
    • Low suction pressure
    • Thermal overload trip
    • Electrical imbalance
    • Repeated short cycling
    • Refrigerant restriction

    Troubleshooting

    1. Check condenser fan operation
    2. Inspect condenser coil cleanliness
    3. Measure refrigerant pressures
    4. Inspect compressor amperage
    5. Verify proper voltage supply

    Correct root cause before resetting the alarm.

    FAQ

    Will resetting clear the fault permanently?
    No. The underlying issue must be corrected.

  • Carrier ComfortLink Alarm 63 – Compressor Lockout Causes

    What Does Alarm 63 Indicate?

    Alarm 63 generally indicates a compressor protection lockout due to repeated fault conditions.

    The controller has detected abnormal operation and has disabled the compressor.

    Common Causes

    • High head pressure
    • Low suction pressure
    • Phase imbalance
    • Overcurrent condition
    • Repeated pressure switch trips

    Troubleshooting

    1. Inspect refrigerant pressures
    2. Check condenser coil cleanliness
    3. Verify fan operation
    4. Inspect electrical connections
    5. Check overload protection devices

    Correct root cause before resetting.

    FAQ

    Will resetting fix it permanently?
    No. The underlying fault must be corrected.

  • Carrier ComfortLink Alarm 31 – Supply Air Sensor Fault Explained

    What Does Alarm 31 Mean?

    Carrier ComfortLink Alarm 31 typically indicates a supply air temperature sensor out-of-range condition.

    The system may:

    • Disable cooling stages
    • Enter protection mode
    • Display unstable temperature readings

    Common Causes

    • Failed thermistor
    • Wiring open circuit
    • Shorted sensor
    • Moisture intrusion
    • Improper sensor placement

    Troubleshooting

    1. Measure sensor resistance
    2. Compare to manufacturer temperature chart
    3. Inspect wiring and terminals
    4. Verify control board voltage reference

    Replace sensor if readings are incorrect.

    FAQ

    Is this similar to Alarm 42?
    Yes, both relate to sensor input faults, though model specifics vary.

  • Carrier ComfortLink Alarm 42 – Causes and Reset Procedure

    What Does Carrier ComfortLink Alarm 42 Mean?

    On Carrier rooftop units and equipment using the ComfortLink control system, Alarm 42 typically indicates a supply air temperature sensor fault or an invalid temperature input being received by the controller.

    When this alarm is active, the unit may:

    • Shut down heating or cooling
    • Lock out compressors
    • Operate erratically
    • Fail to maintain space temperature

    In most cases, Alarm 42 is caused by a failed sensor or wiring issue, not a major equipment failure.

    Common Causes of ComfortLink Alarm 42

    The most common causes include:

    • Failed supply air temperature sensor (thermistor)
    • Loose or damaged sensor wiring
    • Shorted sensor leads
    • Open circuit in the sensor
    • Corroded terminal connections
    • Moisture intrusion at the sensor or control board
    • Failed input channel on the control board (less common)

    Sensor and wiring issues account for the majority of Alarm 42 faults.

    How to Troubleshoot Carrier Alarm 42

    Follow these steps in order.

    1. Power Down the Unit

    Turn off power at the disconnect before inspecting sensors or wiring.

    2. Locate the Supply Air Temperature Sensor

    This sensor is typically mounted in the supply air stream downstream of the evaporator coil.

    3. Inspect Wiring and Connections

    Check for:

    • Loose terminals
    • Corrosion
    • Broken conductors
    • Pinched or rubbed insulation

    Correct any visible wiring issues first.

    4. Measure Sensor Resistance

    Using a digital multimeter, measure the resistance of the sensor and compare it to Carrier’s temperature/resistance chart for that model.

    Common findings:

    • Infinite resistance → open circuit
    • Near-zero resistance → shorted sensor
    • Incorrect resistance → failed sensor

    5. Check Control Board Input

    If the sensor tests correctly, verify reference voltage and input response at the ComfortLink control board.

    Only suspect board failure after confirming the sensor and wiring are good.

    How to Reset ComfortLink Alarm 42

    Once the issue is corrected:

    1. Restore power to the unit
    2. Clear the alarm through the ComfortLink interface if available
    3. If required, cycle unit power to reset the alarm
    4. Verify normal operation and temperature readings

    Some systems will automatically clear the alarm once valid sensor data is restored.

    When to Repair vs Replace

    Replace the sensor if:

    • Resistance readings are incorrect
    • The sensor is physically damaged
    • Moisture contamination is present

    Repair wiring if:

    • Connections are loose
    • Insulation is damaged
    • Corrosion is visible

    Control board replacement should only be considered after all other causes are eliminated.

    Tools That Help

    Technicians commonly use:

    • Digital multimeter with resistance measurement
    • Manufacturer temperature/resistance charts
    • Replacement Carrier-compatible temperature sensor
    • Wiring repair kit

    FAQ

    Can the unit run with Alarm 42 active?
    Usually no. Most systems will disable heating or cooling to prevent improper operation.

    Is Alarm 42 dangerous?
    No, but it can cause loss of cooling or heating if not addressed.

    How much does it typically cost to fix?
    Sensor replacement is typically low cost compared to major component repairs.

    Does Alarm 42 always mean a bad control board?
    No. Sensor or wiring faults are far more common than board failure.