Hold the nuance precisely: the board is reporting a flame signal, and signals can lie — a failing sensor or a board fault can cry flame. But the honest possibilities include a gas valve that is leaking through or closing slowly, and no website gets to tell you to gamble on the sensor explanation. Trane's commercial troubleshooting literature describes the same code the same way: flame sensed with the gas valve not energized, with the combustion fan running hard — the furnace trying to purge a problem it cannot fix.
There is no safe-checks box on this page on purpose. The checks for a flame code are a technician's: valve shutoff integrity, sensor behavior, board logic, in that order, with the gas off when needed.
5 flashes: Flame sensed when no flame should be present
Safety stop — furnace off, call a technician
The control believes there is flame with no call for it. Gas-valve territory.
From the table: Red-LED flash codes — integrated furnace control — source: Trane UC Series Installer's Guide 18-CD19D5-10, diagnostic codes (read the document, fetched 2026-10-30). Covers: 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.
Say the code when you call
"Five flashes on a Trane" changes the dispatch. It tells the office this is not a no-heat-with-a-clean-filter visit; it is a gas-side fault, and it should be scheduled like one. If anyone in the house smells gas at any point, the order changes again: out of the house first, utility or 911 from outside, technician after.
Safety stop — furnace off, call a technician — the safe list:
- Turn the furnace off at its power switch and leave it off.
- This code means the control thinks flame or gas is present when it should not be, or that a flame-safety switch has opened. Resetting it is not a fix.
- Call a licensed HVAC technician. If you smell gas at any point: leave the house and call your gas utility or 911 from outside.
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.
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.