Trane furnace E05: Flame detected when it should not be present

E05 means the control is reading a flame at a time when the gas valve should be closed and nothing should be burning. It points at a gas valve passing gas, or at a sensing circuit reporting a flame that is not there.

Applies to: Trane and American Standard S9V2 two-stage condensing variable speed gas furnaces with a seven-segment integrated furnace control display, per Installer's Guide 18-CE01D1-1E-EN.

What it means

The control checks the flame sensor during windows when no flame should exist. Detecting one in that window has two possible explanations and one of them is serious. Either the gas valve is leaking through when commanded shut, which puts gas into a hot chamber with no controlled ignition, or the sensing circuit is shorted and reporting a flame that is not present. This control has several related codes that narrow it down — E7.1 reports the first stage gas valve energised when it should be off, which is an electrical fault rather than a mechanical leak. But E05 on its own does not distinguish a leaking valve from a shorted sensor, and the safe assumption is the leak.

Most common causes

A gas valve leaking through when it is commanded closed.
A short in the flame sensing circuit reporting a flame that is not present.
A burner still glowing hot enough after shutdown to produce a signal.
Damaged or shorted flame sensor wiring.
A failed control board misreading the sense input.

How to diagnose it

Do not investigate this with the furnace running. Shut the gas off at the manual valve.
If there is any smell of gas, leave the building and call the gas utility from outside.
Check the Last 6 Faults menu for a stored E7.1, which would point at the valve being energised rather than leaking.
Everything beyond shutting off the gas belongs to a licensed technician.

How to fix it

Shut off the gas supply to the furnace.
Call a licensed technician. This is not a homeowner repair under any circumstances.
Expect the gas valve to be checked for leak-through and the sense circuit checked for a short.
A furnace that has recorded this should not be left running while it is investigated.

Notes

The Trane flash code system reports the same condition as five flashes. The meanings agree between Trane's two systems here, which is not something to rely on generally.

All codes for this manufacturer

Trane and American Standard furnace flash codes — every code, with what differs between models.

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-CE01D1-1E-EN — S9V2 two-stage condensing variable speed furnaces, Integrated Furnace Control Display 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.