Trane furnace 5 flashes: Flame sensed when no flame should be present

Five flashes means the control is detecting flame at a time when the gas valve should be closed. Either gas is burning when it should not be, or the sensing circuit is reporting a flame that is not there.

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

The control watches the flame sensor continuously, not only during the burn. Detecting flame while the gas valve is commanded closed produces this code. It is treated seriously because one of the two possible causes is a gas valve that is not fully closing — gas entering the combustion chamber outside a controlled cycle. The other is electrical: a short in the flame sensor wiring produces a signal the control cannot distinguish from real flame. Since it cannot tell them apart, it assumes the dangerous one, which is the correct behaviour for a control managing a gas appliance.

Most common causes

A gas valve leaking through or not closing fully, leaving a small flame after the cycle ends.
A slow-closing gas valve, where flame lingers after shutdown.
A short to ground in the flame sensor wiring or at the sensor.
A chafed or damaged sensor wire touching the cabinet.
A failed control board misreading the flame signal.
Residual flame on a burner after the main flame is cut.

How to diagnose it

Note whether the inducer runs continuously without the furnace heating. Many controls purge the heat exchanger in this condition.
Do not open the burner compartment while the furnace is powered. There is no safe homeowner check for this fault.
Note any smell of gas — if there is one, act on the fix below immediately rather than continuing to diagnose.

How to fix it

If you smell gas: leave the building, operate no electrical switches, and call your gas utility's emergency line or the fire service from outside. Do not return to investigate.
With no gas smell, switch the furnace off at its service switch and leave it off until a licensed technician has seen it.
This fault is entirely technician work. It involves the gas valve and the flame-sensing circuit.
Do not reset repeatedly to see whether it clears. If a valve is not closing, each reset is another opportunity for uncontrolled gas.

Notes

The wiring-short version is far more common than a leaking gas valve, and a technician usually finds it quickly. That does not make it safe to assume, because the control cannot distinguish them and neither can anyone without test equipment at the valve.

7 flashes — Gas valve circuit error

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 5 flashes means on every brand puts them side by side.

This code is not a homeowner repair. Faults in this group involve the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. Do not reset it repeatedly to keep the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on.

Sources

Trane Installer's Guide 18-CD29D1-14-EN — Table 21, Integrated Furnace Control Error Flash Codes

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.