Trane XR90 Furnace Error Codes — Fault Code Diagnostic Guide

Trane XR90 Furnace Error Codes — What They Mean

The Trane XR90 is a single-stage 90% AFUE gas furnace with a PSC (permanent split capacitor) blower motor. It is one of Trane’s long-running mid-efficiency residential furnaces designed for reliability and straightforward service. The XR90 uses a control board with a status LED that reports fault codes via flash sequences. The board also includes a fault history LED window on some revisions that displays the last five fault codes as sticky memory.

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Trane XR90 LED Flash Code Reference

Common Causes by Code

• 2 flashes — Pressure switch open — Blocked condensate drain line (the XR90 is a 90% unit with a condensate system), cracked pressure switch tubing, or failed inducer motor not creating sufficient draft. Check the condensate trap for blockage first — this is the most common cause on XR90s over 5 years old.
• 3 flashes — Pressure switch stuck closed — A pressure switch that closes before the inducer starts indicates a failed switch or a stuck switch from condensate in the switch. Disconnect the tubing from the pressure switch and blow it out.
• 4 flashes — Open limit — Airflow restriction causing heat exchanger overtemperature. Replace the air filter. Check for closed supply registers or a blocked return air path.
• 5 flashes — Flame without call — The flame sensor is detecting signal when the gas valve is off. This indicates a shorted or contaminated flame sensor creating a leakage current, or a failed gas valve that isn’t fully closing.
• 8 flashes — Ignition failure — The XR90 uses a hot surface igniter. Confirm gas supply is on, igniter glows red-orange (not dim yellow), and gas valve opens on ignition call (listen for the click of the valve solenoid).
• 9 flashes — Inducer fault — Inducer motor failed or speed too low. Check inducer capacitor (usually 5–7.5 µF) with a capacitor meter. A seized bearing is the other common cause — spin by hand with power off.

Step-by-Step Fix {#fix}

1. Count the LED flashes — The XR90 LED flashes a code, pauses, then repeats. Count the flashes in one cycle (from pause to pause). The code is the flash count.
2. For 2 flashes (pressure switch open) — Check the condensate drain trap. On the XR90, the trap is usually a U-shaped PVC assembly under the secondary heat exchanger. Remove and flush with water. Also check the condensate hose from the secondary heat exchanger to the trap for kinks.
3. For 4 flashes (limit) — Replace the filter immediately. If filters were just changed, check for a blocked or partially collapsed return air duct. Measure blower wheel RPM if possible — a failing PSC capacitor drops blower speed and reduces airflow.
4. For 8 flashes (ignition failure) — Observe the ignition sequence: inducer starts (verify), igniter heats (glow visible through sight glass), gas valve opens, flame appears. If the flame lights then goes out within 5 seconds, the flame sensor needs cleaning.
5. For 9 flashes (inducer) — Measure supply voltage to the inducer motor (115VAC). Check the capacitor. If voltage is present, capacitor is good, and the motor won’t spin, the motor windings have failed.

Parts Often Needed

When to Call a Pro

The XR90 heat exchanger is a clamshell design that can crack at the secondary (condensate-side) section. If you smell combustion gases in the conditioned air, suspect a cracked heat exchanger — this is a safety issue requiring professional inspection and likely furnace replacement. Do not operate a furnace with a suspected cracked heat exchanger.

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