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.