Furnace lights and displays that are not a fault
A light on a furnace control board is not automatically a fault. Every manufacturer uses the same light to report that the furnace is running normally, and 33 of the codes on this site mean nothing is wrong.
Why a blinking light is usually good news
Most furnace controls use one light for two jobs. When there is no fault it blinks a steady, even heartbeat — typically about one second on and one second off — to say the board is powered and its self-checks are passing. A board showing nothing at all is the one to worry about, because it usually means no power rather than no fault.
The heartbeat is the single most misread signal on a furnace. It is easy to join a slow, even blink part way through and count it as a fault code, and easy to read a light that never stops as a permanent alarm. The test is whether the pattern is even: fault codes come in bursts of a fixed number separated by a longer pause, and a heartbeat has no bursts and no pause.
Displays that show status rather than a fault
Modulating and variable-speed furnaces have a small screen instead of a single light, and most of what it shows is ordinary status: which stage is running, what percentage of full heat is being called for, what the blower is doing. A two-character display showing a letter is far more often reporting the mode than reporting a problem.
Every code on this site that means nothing is wrong
33 codes across 10 control families.
| Brand | What you are seeing | What it means |
|---|---|---|
| Armstrong | Heartbeat | Control powered, no errors detected |
| Carrier | Status code 11 | No previous code stored |
| Carrier | Status code 12 | Blower running after power up |
| Goodman | continuous on | Normal operation — no fault stored |
| Goodman | 1 amber flash | Normal operation — first stage heat |
| Goodman | 1 green flash | Normal operation — low stage cooling |
| Goodman | 2 amber flashes | Normal operation — second stage heat |
| Goodman | 2 green flashes | Normal operation — high stage or single stage cooling |
| Goodman | rapid green flashing | Normal operation — fan only, no heating or cooling |
| Goodman | solid green | Standby — powered, no call |
| Goodman | A | Airflow display, followed by the CFM value |
| Goodman | C | Compressor cooling with a single stage compressor |
| Goodman | C1 | Low stage compressor, cooling mode |
| Goodman | d | Dehumidification cooling with a single stage compressor |
| Goodman | d1 | Dehumidification, low stage cooling, communicating mode |
| Goodman | d2 | Dehumidification, high stage cooling, communicating mode |
| Goodman | dF | Defrost demand |
| Goodman | H | Gas heat demand, followed by the demand percentage |
| Goodman | HI | High heat at 100 per cent demand |
| Goodman | LO | Low stage heat |
| Goodman | On | Normal operation |
| Goodman | P | Compressor heating with a single stage compressor |
| Goodman | P1 | Low stage compressor, heating mode |
| Goodman | P2 | High stage compressor, heating mode |
| Heil | Heartbeat | Normal operation — standby or a call for heat |
| Lennox | Alert code 229 | Ignition on high fire — information only |
| Trane | Flashing fast | Normal — call for heat |
| Trane | Flashing slow | Normal — no call for heat |
| Trane | IdL, Xt1, Xt2, aRF and the other status codes | Normal operation — the display is not showing a fault |
| York | 1 amber flash | Normal operation with a call for heat, Emerson boards |
| York | 2 amber flashes | Normal operation with a call for second stage heat |
| York | Amber heartbeat | Normal operation with a call for heat |
| York | Green heartbeat | Normal operation, no call for heat, no active errors |
If your furnace is not actually heating
A normal-operation code and a furnace that is not producing heat is a real combination, and it usually means the fault is not in the furnace control. A thermostat that is not calling, a tripped breaker on the air handler, a closed gas valve upstream, or a fault in equipment the furnace cannot see will all leave the control reporting that it is fine, because from its point of view it is. Check that the thermostat is calling for heat before treating the furnace as the problem.
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)
- 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
- Trane Installer's Guide 18-CE01D1-1E-EN — S9V2 two-stage condensing variable speed furnaces, Integrated Furnace Control Display Codes and Fault Code Recovery
- Johnson Controls Unitary Products technical service letter ST-045-09 — York UTEC furnace boards, fault code tables and retrieval procedure