Trane furnace Continuous off: Check power
A dark LED means the control has no power. Trane's documented instruction is to check power, and a furnace has several deliberate ways of removing its own.
Applies to: Trane and American Standard upflow, downflow and horizontal gas-fired direct vent condensing furnaces covered by Installer's Guide 18-CD29D1-14-EN, using the integrated furnace control described there.
What it means
The LED is powered by the control board, so a dark LED means the board is not running. Before assuming a failure, it is worth knowing how many ways a furnace legitimately cuts its own power. The blower door switch removes 115 volts whenever the panel is off — which means removing the panel to look at the LED is what turns the LED off, a genuinely confusing design if you have not met it before. Beyond that there is a service switch that looks like a light switch, a dedicated breaker, and a low voltage fuse on the board, any of which produces the same dark indication.
Most common causes
- The blower compartment door is off or not seated, opening the door switch.
- The furnace service switch has been turned off — it looks like an ordinary light switch and is often mistaken for one.
- The circuit breaker for the furnace has tripped or been switched off.
- The low voltage fuse on the control board has blown, usually because of a short in the thermostat wiring.
- A failed transformer, so there is no 24 volt supply.
- A genuinely failed control board.
- A fault in the 115 volt supply to the furnace.
How to diagnose it
- Check the service switch near the furnace first. It is the most common answer and takes a second.
- Check the breaker panel for a tripped furnace breaker.
- Understand the door switch before concluding anything. If your furnace has a sight glass, read the LED with everything closed.
- Note whether the thermostat display is blank, which points at the low voltage side.
- Note whether the breaker trips again immediately after being reset — that is a fault, not an inconvenience.
How to fix it
- Turn the service switch back on, or reset the breaker, if either was off.
- Refit the blower door properly. A door not seated holds the switch open and the furnace will not run.
- If the breaker trips again immediately, stop and call a technician.
- A blown control fuse, failed transformer, or failed board is technician work. A blown fuse in particular usually means a short in the low voltage wiring, and replacing it without finding the short blows the next one.
- If the furnace has been wet, do not power it up. Call a technician.
Notes
The door switch catches nearly everyone at least once. You take the panel off to read the diagnostic light, and taking the panel off is what extinguishes it.
Related
All codes for this manufacturer
Trane and American Standard furnace flash codes — every code, with what differs between models.