HVAC Error Library

Category: Trane Errors

  • Trane RTU Error Code 91 – Communication Fault Explained

    What Does Trane RTU Error Code 91 Mean?

    Error Code 91 on many Trane rooftop units indicates a communication failure between the main controller and a connected module or sensor network.

    When active, the unit may:

    • Show module offline errors
    • Disable cooling or heating stages
    • Operate in limited fallback mode

    Common Causes

    • Loose or damaged communication wiring
    • Incorrect module addressing
    • Failed communication board
    • Electrical interference
    • Grounding issues

    Troubleshooting Steps

    1. Inspect communication wiring for polarity and continuity
    2. Verify unique module addresses
    3. Power cycle affected modules
    4. Check termination resistors if applicable

    Replace control modules only after verifying wiring and addressing.

    FAQ

    Does this mean the RTU has failed?
    No. Most cases involve wiring or configuration issues.

  • Trane RTU Error Code 71 – What It Means and How to Fix It

    What Does Error Code 71 Indicate?

    Trane RTU Error Code 71 commonly indicates a discharge air temperature limit fault or high temperature protection event.

    The unit has detected temperatures outside of safe operating range.

    Common Causes

    • Failed discharge air temperature sensor
    • Blocked condenser coil
    • Inadequate airflow
    • Refrigerant overcharge
    • Stuck economizer damper
    • Faulty limit switch

    Troubleshooting Steps

    1. Inspect Condenser Coil

    Dirty coils can cause excessive temperature buildup.

    Clean if necessary.

    2. Verify Sensor Accuracy

    Measure sensor resistance and compare to manufacturer chart.

    Replace if out of range.

    3. Check Airflow

    Confirm fan operation and unobstructed airflow paths.

    4. Inspect Refrigerant Pressures

    Overcharge or restriction can elevate discharge temperature.

    FAQ

    Is this related to high head pressure?
    Often yes. High head pressure conditions can trigger temperature protection.

  • Trane RTU Error Code 45 – Causes and Troubleshooting

    What Does Trane RTU Error Code 45 Mean?

    On many Trane rooftop units, Error Code 45 indicates a supply fan proving switch fault or airflow verification failure. The controller is not receiving confirmation that the supply fan is operating as expected.

    When this fault occurs, the unit may:

    • Prevent compressor operation
    • Disable heating or cooling stages
    • Enter lockout mode

    Common Causes of Error Code 45

    Typical causes include:

    • Failed airflow proving switch
    • Broken or loose tubing to pressure switch
    • Dirty filters
    • Failed supply fan motor
    • Blower belt failure (belt-driven units)
    • Faulty fan contactor
    • Control board input issue

    Airflow restrictions are among the most common causes.

    How to Troubleshoot

    1. Verify Fan Operation

    Confirm the supply fan motor is running when commanded.

    Check:

    • Motor amperage
    • Belt condition
    • Shaft rotation

    2. Inspect Airflow Proving Switch

    Check:

    • Tubing for cracks or blockage
    • Proper pressure differential
    • Electrical continuity

    Replace the switch if it does not close under proper airflow.

    3. Inspect Filters and Coil

    Restricted airflow can prevent the proving switch from closing.

    Replace dirty filters and clean coils as needed.

    When to Replace Components

    Replace the airflow switch if:

    • It fails continuity under proper airflow
    • Tubing is intact but no signal is present

    Only suspect control board failure after verifying mechanical components.

    FAQ

    Will the unit cool with this fault active?
    Usually no. The unit prevents operation without verified airflow.

    Is this a serious fault?
    It protects the equipment from operating without airflow.

  • 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.