All Brands — Universal error codes, explained
Cross-brand furnace and AC fixes every office should know. Codes below: Symptom-based guides. Each guide covers what the code means, what you can safely try yourself, when to stop and call a technician, and what the repair typically costs.
Furnace lights, then dies seconds later — the dirty flame sensor (any brand)
The single most common furnace failure across every brand, and it costs nothing to fix. The flame sensor is a thin metal rod that proves the burners lit; when coated with combustion residue it can't sense the flame, so the board — believing ignition failed — shuts the gas off after a few seconds. Three to five failed tries and the furnace locks out (Carrier Code 34/14, Trane 2 flashes, Lennox Watchguard/E270 — all roads lead here).
AC suddenly won't run at all — check the condensate float switch (any brand)
If the thermostat is calling for cooling but the system is completely dead, there's a decent chance nothing is broken: the condensate drain clogged, water backed up, and the float safety switch cut the system off before the pan overflowed into your ceiling. It's protecting you. The fix is clearing the drain, and it's a 15-minute job.