Furnace flame sensor codes on every brand

Flame sensing faults split into two opposite problems: the furnace lit and could not prove it, or the furnace sees flame when there should be none. They call for completely different responses.

Some of the faults listed here are gas-safety faults. Those ones 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, and each carries this warning on its own page. Do not keep resetting a furnace that reports one of them in order to hold 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. The conditions that mean stop now.

What the flame sensor does

The flame sensor is a single metal rod in the burner flame. The control puts an AC voltage on it and relies on flame rectification — ionised gas in the flame conducts current in one direction only, so the control reads a small DC microamp current back and takes that as proof of flame. A coating of oxide on the rod, which builds up normally over a season, cuts that current without changing anything visible.

Why one part produces several different codes

No flame sensed after ignition makes the control shut the gas valve within seconds and retry, then lock out. Flame sensed when none should be present is the more serious of the two: the control is being told there is a flame while the gas valve is shut, which points at a gas valve leaking through, a shorted sensor circuit, or a burner still glowing. Manufacturers treat that as a condition requiring a technician rather than a cleaning.

Every flame sensor code on this site

22 codes across 11 control families. The meaning is the manufacturer's, from the document listed at the foot of each code page.

Brand	Code	What it reports
Armstrong	4 flashes	Low flame signal — check the flame sensor
Armstrong	12 flashes	Flame sensed without the gas valve energised
Carrier	Status code 22	Abnormal flame-proving signal — flame detected with the gas valve closed
Goodman	5 flashes	Flame detected when no flame should be present
Goodman	7 flashes	Flame signal is too weak
Goodman	E4	Flame detected when no flame should be present
Goodman	E6	Flame signal is too weak
Goodman	1 red flash	Flame sensed when no flame should be present
Goodman	rapid amber flashing	Low flame sense current
Heil	5 flashes	Flame sensed out of sequence — flame signal still present
Lennox	Alert code 240	Low flame current in run mode
Lennox	E240	Low flame current in run mode
Lennox	E241	Flame sensed out of sequence — flame still present
Lennox	E270	Soft lockout — retries exceeded, no flame current sensed
Lennox	E275	Soft lockout — flame sensed out of sequence, signal now gone
Trane	5 flashes	Flame sensed when no flame should be present
Trane	8 flashes	Low flame sense signal
Trane	E2.2	Recycles exceeded — flame sensed then lost, one hour lockout
Trane	E05	Flame detected when it should not be present
Trane	E08	Flame current low but still usable
York	1 red flash	Flame sensed with the gas valve off
York	Rapid amber flashing	Low flame sense current

Before you work on it

Flame sensing is the circuit that shuts the gas off when the burner does not light. Work on it belongs to a licensed technician.

What the fixes cost

Flame sensor work is the classic cheap fix: the furnace repair cost guide lists cleaning at $80–200 and calls it a ten-minute job. That number is worth holding onto while reading this page, because the two families of code above lead to completely different bills. The could-not-prove-flame family — weak signal, lost flame, failed trials — usually ends at that $80–200 cleaning, occasionally at a grounding repair found during a $75–150 diagnostic. The flame-when-there-should-be-none family is different money and different urgency: if the cause is a gas valve not closing fully, the valve runs $300–900 and is never a DIY part. Same sensor, same microamps — opposite ends of the price sheet.

Do it yourself or call someone?

Neither family is homeowner work, for the reason the top of this page gives: this is the circuit that decides whether gas keeps flowing. What is homeowner work is triage. The weak-signal warning codes — the furnace still heats — justify booking a normal-hours visit at your convenience. The flame-out-of-sequence codes justify shutting the furnace off at the service switch and calling promptly, and any smell of gas means leaving the building and calling the gas utility from outside. Knowing which family your code belongs to is most of the value of this page.

Other parts

Sources

Every code meaning above comes from the manufacturer document behind its code page:

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.