Heil furnace 5 flashes: Flame sensed out of sequence — flame signal still present

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

Applies to: International Comfort Products *9MPT and *9MPV gas furnaces using the Honeywell SV9541Q SmartValve, sold as Heil, Tempstar and Comfortmaker. On the other ICP diagnostic system, five flashes means a high pressure switch fault instead — confirm which light you are reading.

What it means

ICP's wording is precise: flame sensed out of sequence, flame signal still present. The control watches the flame sensor continuously, not only during the burn, and detecting flame outside the expected part of the cycle 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 sequence. The other is electrical: a short in the flame sensing circuit produces a signal the control cannot distinguish from a real flame. Since it cannot tell them apart, it assumes the dangerous one, which is correct behaviour for a control managing a gas appliance.

Most common causes

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

How to diagnose it

Note any smell of gas and act on it immediately rather than continuing to diagnose.
Do not open the burner compartment while the furnace is powered. There is no safe homeowner check for this fault.
Confirm you are reading the gas valve LED. On the ICP control board system, five flashes means a high pressure switch fault, which is an entirely different and less urgent problem.
Note whether the inducer runs continuously without the furnace heating, which can accompany this condition.

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 go back in to investigate.
With no gas smell, turn 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 — the two parts of a furnace a homeowner should never work on.
Do not reset repeatedly to see whether it clears. If a valve is passing gas, each reset is another opportunity for it to accumulate.

Notes

This is the code where the two ICP diagnostic systems diverge most dangerously. Five flashes on the control board system is a draft problem worth investigating at leisure. Five flashes here is a possible gas valve fault. Reading the wrong table could send someone to check their vent termination while gas is passing.

All codes for this manufacturer

Heil, Tempstar and Comfortmaker furnace 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

International Comfort Products *9MPT and *9MPV installation instructions — Gas Valve Diagnostic Codes (third-party archive of the ICP document)

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.