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.

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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.

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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.

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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.

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Tools That Help

Technicians commonly use:

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

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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.