Furnace dead: no lights on the control board
No diagnostic light, no inducer, no response to the thermostat. Before assuming the board has failed, there are four things that produce exactly this and are not a failed board.
What is actually happening
Every furnace control board has a status LED that shows something — a steady light, a slow flash, or a heartbeat — whenever the board has power. A board showing nothing at all is not reporting a fault code, because reporting requires power. It is telling you power is not arriving, or the board itself is dead.
What causes it
The four common causes, roughly in order: the blower door safety switch is not made, because a panel was left off or is not seated, and that switch cuts all power by design; the furnace disconnect switch, which looks exactly like a light switch and is often near the unit, is off; the circuit breaker has tripped; or the low voltage fuse on the board, usually a 3 or 5 amp automotive-style blade fuse, has opened. That fuse blows for a reason — most often a shorted low voltage wire, a shorted thermostat wire, or a failed transformer — so replacing it without finding the short simply blows the next one.
The codes that report this
6 codes across 5 control families. Each links to what that manufacturer says about it.
| Brand | Code | What it reports |
|---|---|---|
| Armstrong | LED off | No power to the control, or a control hardware fault |
| Carrier | LED continuously off | No power reaching the control |
| Carrier | Status code 24 | Secondary voltage fuse is open |
| Goodman | LED off | No power to the furnace, or an internal control fault |
| Goodman | E5 | Low voltage fuse is open |
| York | Steady off | No power to the board, or control failure |
Worth knowing
If the fuse is intact, the disconnect is on, the breaker is on and the door switch is made, the remaining possibilities are the transformer and the board. Both are measurements rather than inspections, and belong to a technician.
Other symptoms
- Lights, then shuts off after a few seconds
- Igniter glows but the burners never light
- Runs for a while, then trips on high limit
- Inducer runs but the burners never come on
- Runs briefly, stops, starts again, then quits
Sources
- Allied Air Enterprises — A80UH1E and 80G1UHE Installation Instructions 507330-03, Table 7: Diagnostic Codes for -02 Control
- Carrier 58MCB Service Manual, 327884-101 Rev. B — status codes
- Goodman Manufacturing — Diagnostic Code Identification System (model-specific fault code lookup)
- Johnson Controls Unitary Products technical service letter ST-045-09 — York UTEC furnace boards, fault code tables and retrieval procedure