Carrier furnace LED continuously off: No power reaching the control
A dark status LED means the control has no power. Carrier's documented check is for 115 volts at L1 and L2, and 24 volts at SEC-1 and SEC-2.
Applies to: Carrier 58MCB four-way multipoise condensing gas furnaces and related models using the same integrated furnace control. Bryant and Payne furnaces of the same generation use the same control and code set.
What it means
The status LED is powered by the control board, so a dark LED means the board is not running. Carrier gives the specific test points: 115 volts across L1 and L2 for the line supply, and 24 volts across SEC-1 and SEC-2 for the secondary. Those two measurements split the problem cleanly — no 115 volts means the fault is upstream in the house wiring or the furnace's own switches, while 115 volts present but no 24 volts means the transformer or the fuse. Before any of that, though, a furnace has several deliberate ways of removing its own power, and those are worth eliminating first because they need no tools.
Most common causes
- The blower compartment door is off or not seated, opening the door switch and cutting 115 volt power.
- The furnace service switch has been turned off — it looks like an ordinary light switch.
- The circuit breaker for the furnace has tripped or been switched off.
- The secondary voltage fuse has blown, which may also present as code 24 when the board is powered.
- A failed transformer, so there is no 24 volt supply.
- A 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. Removing the blower door to look at the LED is what turns the LED off.
- Note whether the thermostat display is blank. A furnace-powered thermostat going dark points at the low voltage side.
- Note whether the breaker trips again immediately after being reset, which is a fault rather than 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.
- Voltage testing at L1, L2, SEC-1 and SEC-2 is technician work — those are live terminals.
- If the furnace or the area around it has been wet, do not power it up at all. Call a technician.
Notes
The door switch catches almost everyone once. You remove the panel to read the LED, and removing the panel is what extinguishes it. Read codes through the sight glass with the furnace closed where one is fitted.