Furnace flame sensor codes on every brand
Flame sensing faults split into two opposite problems: the furnace lit and could not prove it, or the furnace sees flame when there should be none. They call for completely different responses.
What the flame sensor does
The flame sensor is a single metal rod in the burner flame. The control puts an AC voltage on it and relies on flame rectification — ionised gas in the flame conducts current in one direction only, so the control reads a small DC microamp current back and takes that as proof of flame. A coating of oxide on the rod, which builds up normally over a season, cuts that current without changing anything visible.
Why one part produces several different codes
No flame sensed after ignition makes the control shut the gas valve within seconds and retry, then lock out. Flame sensed when none should be present is the more serious of the two: the control is being told there is a flame while the gas valve is shut, which points at a gas valve leaking through, a shorted sensor circuit, or a burner still glowing. Manufacturers treat that as a condition requiring a technician rather than a cleaning.
Every flame sensor code on this site
15 codes across 9 control families. The meaning is the manufacturer's, from the document listed at the foot of each code page.
| Brand | Code | What it reports |
|---|---|---|
| Armstrong | 12 flashes | Flame sensed without the gas valve energised |
| Armstrong | 4 flashes | Low flame signal — check the flame sensor |
| Carrier | Status code 22 | Abnormal flame-proving signal — flame detected with the gas valve closed |
| Goodman | 5 flashes | Flame detected when no flame should be present |
| Goodman | 7 flashes | Flame signal is too weak |
| Goodman | E4 | Flame detected when no flame should be present |
| Goodman | E6 | Flame signal is too weak |
| Goodman | 1 red flash | Flame sensed when no flame should be present |
| Goodman | rapid amber flashing | Low flame sense current |
| Heil | 5 flashes | Flame sensed out of sequence — flame signal still present |
| Lennox | Alert code 240 | Low flame current in run mode |
| Trane | 5 flashes | Flame sensed when no flame should be present |
| Trane | 8 flashes | Low flame sense signal |
| York | 1 red flash | Flame sensed with the gas valve off |
| York | Rapid amber flashing | Low flame sense current |
Before you work on it
Flame sensing is the circuit that shuts the gas off when the burner does not light. Work on it belongs to a licensed technician.
Other parts
- Furnace pressure switch codes on every brand
- Furnace high limit switch codes by brand
- Furnace rollout switch codes: why it tripped
- Furnace igniter fault codes by manufacturer
- Furnace inducer motor fault codes by brand
- Furnace gas valve fault codes by manufacturer
- Furnace grounding and reversed polarity codes
Sources
Every code meaning above comes from the manufacturer document behind its code page:
- 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)
- Goodman Technical Services Service Bulletin SF-042 — Gas Valve Diagnostics
- International Comfort Products *9MPT and *9MPV installation instructions — Gas Valve Diagnostic Codes (third-party archive of the ICP document)
- Lennox Residential Communicating Systems Alert Code Guide, document 100017, March 2023 — Table 2, Alert Codes and Troubleshooting
- Trane Installer's Guide 18-CD29D1-14-EN — Table 21, Integrated Furnace Control Error Flash Codes
- Johnson Controls Unitary Products technical service letter ST-045-09 — York UTEC furnace boards, fault code tables and retrieval procedure