Trane error codes, explained
Trane gas furnaces and light-commercial units (American Standard shares the platform). Codes below: Status-LED flash codes (2–5 flashes…). Each guide covers what the code means, what you can safely try yourself, when to stop and call a technician, and what the repair typically costs.
Trane furnace 2 flashes — External lockout (retries exceeded)
The board gave up after repeated failed ignition attempts and locked out — Trane's legend calls it 'external lockout.' On light-commercial equipment Trane documents this as a 1-hour lockout that auto-clears. The classic root cause is a dirty flame sensor.
Trane furnace 3 flashes — Pressure switch error
The draft pressure switch is open when it should be closed (or closed when it should be open). Causes: blocked vent or intake (check the exterior termination — snow and ice are seasonal classics), a failing draft inducer, or on condensing furnaces a blocked condensate drain.
Trane furnace 4 flashes — High-temperature limit open
The furnace overheated and the limit device opened. Nearly always restricted airflow: clogged filter, blocked returns, closed registers, or a dirty blower/coil.
Trane furnace 5 flashes — Flame sensed when it shouldn't be
The board detects flame when no flame should be present — for example, a gas valve that isn't closing fully or a lingering flame after shutdown. Unlike the other flash codes, this is not a DIY item: out-of-sequence flame implies a possible gas-valve failure.
How to read the Trane furnace status light
On most Trane/American Standard furnace boards the diagnostic LED is RED, and the 'normal' states confuse everyone: SLOW flashing = normal standby (no call for heat), FAST flashing = normal operation with a call for heat. Counted groups of flashes (2, 3, 4, 5…) are fault codes. Steady ON = replace the board; steady OFF = no power. (Some board families — including Trane's light-commercial ReliaTel and certain models — use a green LED with similar logic, but 'flashing means broken' is the myth to unlearn: flashing is usually normal.)