Trane furnace 8 flashes: Low flame sense signal
Eight flashes means the furnace is lighting and running, but the control can barely detect the flame. It is a warning ahead of a failure rather than a failure itself.
Applies to: Trane and American Standard upflow, downflow and horizontal gas-fired direct vent condensing furnaces covered by Installer's Guide 18-CD29D1-14-EN, using the integrated furnace control described there.
What it means
Flame detection works by rectification: the control applies a small voltage to the flame sensor and reads the tiny current flowing through the flame, measured in microamps. This code means that current is above zero but low enough to flag. That makes it genuinely useful — a predictive fault. The furnace is still working while it is displayed. Left alone, the signal keeps falling until it drops below the threshold, at which point the furnace can no longer prove flame, shuts the gas off, and eventually locks out with two flashes instead. Acting on it while the furnace still heats is both cheaper and considerably more convenient.
Most common causes
- The flame sensor is coated with oxide or carbon, insulating it from the flame. This is the most common cause by a wide margin.
- The sensor is bent or has shifted so it sits only partly in the flame.
- Poor furnace grounding, since rectification needs a solid ground path.
- A weak or lazy burner flame caused by low gas pressure.
- Restricted combustion air, producing a flame that carries current poorly.
- A cracked ceramic insulator on the sensor.
- Corroded or loose wiring between the sensor and the control.
How to diagnose it
- Note that the furnace is probably still heating. This code often appears while everything seems fine, which is exactly when acting on it is cheapest.
- Check whether the furnace has started short-cycling or occasionally failing to stay lit — that is the signal falling further.
- Check when it was last serviced. A sensor uncleaned for several seasons is the usual explanation.
- Mention any other intermittent electrical faults to the technician, since poor grounding causes this and others together.
How to fix it
- Book a service call before the furnace fails rather than after. The repair is usually inexpensive.
- Cleaning a flame sensor is technician work. It sits in the burner flame, in the circuit that decides whether gas keeps flowing.
- Have grounding checked at the same visit if the sensor was cleaned recently and the code has come back.
- Low gas pressure needs correcting at the valve or the supply, which is licensed work.
Notes
A flame sensor is a plain steel rod with a ceramic insulator and no moving parts, and cleaning it takes minutes. The alternative is the same repair on the coldest night of the year at emergency rates.
Related
All codes for this manufacturer
Trane and American Standard furnace flash codes — every code, with what differs between models.
Not sure this is your brand?
The same count means something else on other furnaces. What 8 flashes means on every brand puts them side by side.