HVAC Error Library

Author: scott

  • 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.

  • Trane RTU Error Code 79 – What It Means and How to Fix It

    What Does Error Code 79 Indicate?

    Trane RTU Error Code 79 commonly indicates a communication fault between the unit controller and a connected module or sensor.

    The unit may:

    • Enter fault mode
    • Display intermittent operation
    • Show module offline alarms

    Common Causes

    • Loose communication wiring
    • Damaged control harness
    • Module failure
    • Address conflict
    • Control board malfunction
    • Grounding issues

    Troubleshooting Steps

    1. Inspect Communication Wiring

    Check:

    • RS-485 polarity
    • Shield grounding
    • Loose terminals

    2. Verify Addressing

    Ensure unique module addresses.

    3. Power Cycle Modules

    Shut down and restore power to clear transient faults.

    4. Replace Faulted Module (If Confirmed)

    Only after wiring and addressing are verified.

    FAQ

    Does this require replacing the RTU?
    No. Most cases involve wiring or module issues.

  • Trane RTU Error Code 33 – Causes and Troubleshooting

    What Does Trane RTU Error Code 33 Mean?

    On many Trane rooftop units, Error Code 33 indicates a low-pressure switch fault or refrigerant circuit protection issue. The unit has detected abnormal suction pressure and has shut down the compressor to prevent damage.

    When this code is active, the unit may:

    • Disable cooling
    • Lock out the compressor
    • Require manual or power reset

    Common Causes of Error Code 33

    Typical causes include:

    • Low refrigerant charge
    • Refrigerant leak
    • Failed low-pressure switch
    • Dirty evaporator coil
    • Restricted airflow
    • Faulty pressure transducer
    • Defective compressor contactor

    Low charge and airflow problems are the most common field causes.

    How to Troubleshoot

    1. Verify Airflow

    Check:

    • Filters
    • Evaporator coil cleanliness
    • Blower operation
    • Belt tension (if applicable)

    Restricted airflow can drop suction pressure.

    2. Inspect Refrigerant Charge

    Check system pressures and compare to manufacturer specifications.

    If pressures are low:

    • Check for leaks
    • Inspect service valves
    • Examine joints and fittings

    3. Test the Low-Pressure Switch

    Using a multimeter:

    • Confirm switch continuity
    • Verify cut-in and cut-out operation

    Replace if defective.

    4. Check Control Board Input

    Verify that the control board is receiving accurate pressure signals.

    When to Replace Components

    Replace the low-pressure switch if:

    • It does not close under normal pressure
    • It shows erratic readings

    Only suspect compressor failure after verifying proper charge and airflow.

    FAQ

    Will the unit reset automatically?
    Some models auto-reset after pressure normalizes; others require manual reset.

    Is this usually a major repair?
    Often not. Many cases are airflow-related.

  • Trane RTU Error Code 91 – What It Means and How to Fix It

    What Does Trane RTU Error Code 91 Mean?

    On most Trane rooftop units (RTUs), Error Code 91 indicates a supply air temperature sensor fault. This typically means the unit control board is receiving an out-of-range or invalid signal from the supply air sensor.

    This can cause:

    • Unit lockout
    • Compressor shutdown
    • Erratic heating or cooling
    • Failure to maintain setpoint

    In most cases, this is a sensor wiring issue or failed thermistor — not a major mechanical failure.

    Common Causes of Error Code 91

    The most frequent causes include:

    • Failed supply air temperature sensor
    • Loose or corroded sensor wiring
    • Shorted sensor leads
    • Open circuit in sensor wiring
    • Control board input failure (less common)
    • Moisture intrusion at terminal connections

    Sensor failure is far more common than board failure.

    How to Troubleshoot Trane Error Code 91

    Follow these steps:

    1. Power Down the Unit

    Shut off power at the disconnect before checking wiring.

    2. Locate the Supply Air Sensor

    Typically mounted downstream of the evaporator coil in the supply air section.

    3. Inspect Wiring

    Check for:

    • Loose terminal screws
    • Corrosion
    • Damaged insulation
    • Pinched wires

    4. Measure Sensor Resistance

    Most Trane sensors are thermistors. Compare resistance reading to manufacturer temperature/resistance chart.

    If resistance is:

    • Infinite → open circuit
    • Near zero → shorted
    • Way off expected value → failed sensor

    5. Check Control Board Input

    If sensor tests correctly, verify voltage/reference at the control board input terminals.

    When to Replace vs Repair

    Replace the sensor if:

    • Resistance values are incorrect
    • Physical damage is visible
    • Moisture contamination exists

    Repair wiring if:

    • Loose connections
    • Corrosion
    • Insulation damage

    Only suspect control board failure after verifying sensor and wiring.

    Tools That Help

    Technicians commonly use:

    • Digital multimeter (with resistance measurement)
    • Temperature probe for comparison testing
    • Replacement OEM thermistor
    • Wiring repair kit

    FAQ

    Can I reset Error Code 91?

    Yes. After correcting the issue, cycle power to clear the fault. Some models require manual reset at the control board.

    Will the unit run with this fault active?

    Usually no. Most Trane RTUs will enter lockout mode.

    Is this an emergency?

    It can be if the unit serves critical cooling areas. Otherwise, it’s typically a service-level repair.

    How much does repair usually cost?

    Sensor replacement is generally inexpensive compared to compressor or board replacement.

  • 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.