Furnace Won't Start
Dead furnace call. Easy to skip steps and swap the wrong part. Walk the power path upstream methodically and most of these fall apart in about 10 minutes.
- Line power or door interlock interrupted20-30%
- Lost 24V — blown board fuse, failed transformer, or low-voltage short20-30%
- Thermostat not delivering W (often smart stat with C-wire issue)15-25%
- Open safety string — tripped float, stuck pressure switch, open limit/rollout15-25%
- 1
Prove 120 VAC at the disconnect and at the board L1/neutral with door engaged
Voltage loss between those points is the door interlock or internal wiring. Loose blower doors make a furnace look totally dead.
- 2
Check transformer — 120 VAC primary, 24 VAC secondary
Primary good with no secondary = transformer dead. Find what killed it (shorted accessory, T-stat wiring, float) before replacing.
- 3
Inspect the board low-voltage fuse, then meter R-C for 24 VAC
A blown fuse is a symptom of a short — find the short before the new fuse goes in.
- 4
Meter W-C at the board during a heat call, then jumper R to W to bypass the stat
If R-C is good but W-C is dead during the call, the problem is on the thermostat side. If the furnace runs jumpered, stop chasing furnace parts.
- 5
Read the board code, then voltage-drop across each safety in the string
0 VAC across a closed switch, 24 VAC across the open one — fastest way to find the culprit without breaking connections.
- 6
Check the condensate float and drain
Float switches that break R kill the whole T-stat circuit and look like a dead stat. On some 90+ units the blocked-drain switch is in series with the gas valve.
- 7
Only after all of the above, suspect the board
Power, 24V, fuse, R-C, W call, and safeties all proven good — now a board call is justified, not a guess.
The board is the last suspect, not the first. Don't condemn it until line power, transformer, fuse, R-C, W-C, and the safety string are all proven good.
A blown low-voltage fuse always means there's a short somewhere — drop a new fuse in without finding it and you'll blow the new one too.