Two things make Trane codes harder to read than they should be. First, the green LED is a state light, not a code light: slow flash means no call for heat, fast flash means a call — both normal, both constantly searched as if they were faults. Faults are on the red LED.
Second, the replay: on power-up, the control flashes its last four stored faults, newest first, two seconds apart. Homeowners count one endless pattern and conclude the furnace is flashing 17 times. It is not; it is reading its diary. American Standard furnaces use the same Trane control and the same tables.
The two rules that outrank everything on this page.
1. Smell gas? Stop reading. Leave the house and call your gas utility or 911 from outside. No code is worth diagnosing through a gas smell.
2. Never bypass a safety switch — no jumping a pressure switch, no taping a limit, no propping a rollout. Those switches are the only thing between a furnace fault and a fire or carbon-monoxide problem. This library explains codes; it does not teach repairs inside the cabinet.
Which chart is yours?
The main chart is from Trane’s Installers Guide 18-CD19D5-10 for the UC/AUH furnace families — the IFC most residential Trane and American Standard furnaces of the last two decades carry. Below it, a second short table holds the rows where Trane’s inducer-kit guide (18-CH66D1-5A) differs — codes 9 and 10 especially. If your door chart shows a 10-flash code, you have the second family.
Red-LED flash codes — integrated furnace control
Trane/American Standard furnaces with the IFC covered by Installers Guide 18-CD19D5-10 (UC/AUH families and relatives). Green LED reads system state; the red LED flashes the faults.
| What you see | What the manual says it means |
|---|---|
| Slow flash (green) | Normal — no call for heat — A slow, lazy blink is the furnace idling happily. People search this constantly; it is not a code. |
| Fast flash (green) | Normal — call for heat — Fast steady flashing on the green LED means the thermostat is asking for heat and the board knows it. |
| LED on (continuous) | Replace the IFC — The installers guide is unusually direct: continuous ON means the integrated furnace control itself has failed. |
| LED off (continuous) | Check power — No light at all: the guide’s entire entry. Switch, breaker, door switch, then a meter. |
| 2 flashes | System lockout — retries or recycles exceeded — The furnace tried to light or stay lit too many times and locked out. On power-up this board replays its last 4 faults, newest first, with a 2-second gap between codes — count carefully or you will merge two codes into one. Full page → |
| 3 flashes | Draft pressure error — The guide gives exactly three suspects: a venting problem, a pressure switch problem, or an inducer problem. That order is a good troubleshooting order, too. Full page → |
| 4 flashes | Open temperature limit circuit — The furnace overheated and the limit opened the circuit. Airflow first, always. Full page → |
| 5 flashes | Flame sensed when no flame should be present — The control believes there is flame with no call for it. Gas-valve territory. Full page → |
| 6 flashes | 115VAC power reversed, poor grounding, or system voltage too low — Three electrical suspects in one code. The board cannot trust its flame signal without a proper ground, which is why this code exists at all. |
| 7 flashes | Gas valve circuit error — The valve circuit is not doing what the board commanded. No homeowner move beyond shutting it down. |
| 8 flashes | Low flame sense signal — The flame is there but the signal is weak — classically a coated flame sensor. A technician cleans or replaces the sensor and measures the microamps. |
| 9 flashes | Check the igniter circuit — From the AUH Installers Guide: check the igniter, and check that line-neutral to 24VAC common reads no more than 2 volts — a grounding problem can hide here. Note: a different Trane document (the inducer-kit guide) uses 9 for an open inducer limit instead. Your door chart decides. |
Source: Trane UC Series Installer's Guide 18-CD19D5-10, diagnostic codes. Read the document (fetched 2026-10-30).
Variant rows — furnaces covered by Trane guide 18-CH66D1-5A
Trane’s KIT16582 inducer-kit installation guide lists a slightly different tail to the same table. Rows 2-5, 7 and 8 match the main chart; these rows differ or are new.
| What you see | What the manual says it means |
|---|---|
| 6 flashes (variant) | 115VAC reversed, igniter (triac) fault, poor grounding, or low voltage — Same code as the main chart, one extra suspect: the igniter triac on the board. |
| 9 flashes (variant) | Open inducer limit switch — On the models this guide covers, 9 is the inducer limit — not the igniter-circuit check in the main chart. This is the single best example on this site of why the chart inside your blower door outranks any website, including this one. |
| 10 flashes | Inducer communication error — The board and the inducer are not talking. A connector/wiring or board problem, not a venting problem. |
Source: Trane KIT16582 Installation Guide 18-CH66D1-5A-EN, IFC flash codes. Read the document (fetched 2026-10-30).
One honesty note before the tables: codes change between control boards, sometimes inside the same brand and the same decade. Every table below names the board and the manual it was transcribed from. The chart glued inside your own blower door is the final word for your furnace — if it disagrees with this page, believe the door.
Questions
Why does my Trane flash codes when I turn the power back on?
That is the fault history replay: the last four faults, newest first, with a two-second pause between them. It is a feature. Wait for the replay to finish; the live fault, if any, is what shows afterward.
Is a slow-blinking Trane light a fault?
No. Slow flash on the green LED is normal idle — no call for heat. Fast flash is also normal: a call for heat. Faults live on the red LED.
Where these meanings come from
Transcribed 2026-10-30 from the manufacturers’ own documents — not from other blogs:
- Trane UC Series Installer's Guide 18-CD19D5-10, diagnostic codes — read it
- Trane KIT16582 Installation Guide 18-CH66D1-5A-EN, IFC flash codes — read it