Heil, Tempstar and Comfortmaker Furnace Error Codes

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the maker’s manual

Heil, Tempstar and Comfortmaker Furnace Error Codes

Short answer

Heil, Tempstar and Comfortmaker furnaces are built by ICP and share its green status light: short flashes give the first number and long flashes the second. 3 is the pressure switch, often a plugged condensate drain or vent, 4 a limit or rollout switch, 6 a failed ignition and 6+1 the lockout after four tries. A solid light means a one hour lockout.

Heil, Tempstar, Comfortmaker, Arcoaire, KeepRite and Day and Night furnaces are all built by International Comfort Products, ICP, and many share the same control. Its green status light, seen through a small window in the blower door, flashes a code when something goes wrong.

Everything on this page comes from ICP’s own Service and Technical Support Manual 440 04 4401 01 for the N9MSE condensing furnace, and the brand list from ICP’s own site. Other ICP models use similar codes, and ICP says the service label on your blower door explains the codes for your furnace.

Before you start

What to carry: a torch and a pen. Ten minutes.

Gas or electric? These are gas furnace codes, natural gas or propane.

How to count. ICP says the code is one or two numbers: the first is the number of short flashes, and the second, after the plus sign, is the number of long flashes. Six short and one long is 6+1.

Leave the door on. The light shows through the window in the blower door. Taking the door off opens a safety switch and cuts power to the control.

Two codes where you stop. 5: flame with the gas valve off. ICP says the control has proved a flame while the gas valve is switched off, and to check for a stuck open or leaky gas valve. Turn the furnace off, turn the gas off to it and call a technician.

4 and 7 from the rollout switch. ICP says the flame rollout switch needs a manual reset. A rollout switch trips because flame came out of the burner box, so find out why before pressing it: what a rollout switch is telling you.

What the green light is doing

ICP's troubleshooting chart starts with one question: is the light pulsing like a heartbeat, on solid, off, or flashing a code?

Check 1Free, two minutes

Read the light

A heartbeat, bright then dim. ICP’s chart treats this as the normal state and moves on to looking for a stored code.

On solid. ICP says the control is locked out and resets itself after one hour. It names a flame sense circuit failure, a gas valve relay stuck open or a software check error, and says to reset the power to clear it and replace the control if it repeats.

Off. No power reaches the control. ICP’s chart checks the breaker, the blower door switch, the 24 volt fuse and the transformer, in that kind of order.

Flashing on and off. A status code. Count it and find it in the table below.

Every code in ICP’s chart

Count the short flashes, then the long ones, up to the pause. The pause is where one code ends.

Code	What ICP says it means	What ICP says to check
1+2	Blower on after power up, normal	Nothing; it runs 90 seconds
2	Pressure switch did not open	An obstructed pressure tube, a stuck switch
3	Pressure switch did not close, or reopened	Vent, condensate drain, wind, tubing, inducer
4	Limit circuit fault, a limit or rollout switch open	Filter, ducts, blower wheel, gas input
5	Flame proved with the gas valve off	A stuck open or leaky gas valve
6	Ignition proving failure	Gas turned on, flame sensor, ground wire
6+1	Ignition lockout after 4 tries	As 6; resets itself after 3 hours
7	Limit circuit lockout, open over 3 minutes	As 4; resets itself after 3 hours
8	Gas heating lockout, no automatic reset	Power off 5 minutes, then the gas valve relay
10	Polarity	Line voltage wired the wrong way round
On solid	Control locked out	Resets after 1 hour, or cut the power

The codes you can often clear yourself

Check 2

Work the free checks

3: the pressure switch. ICP’s list includes a plugged condensate drain, water in the vent piping from a sagging pipe, a restricted vent and excessive wind. Look at the pipes outside for snow, ice or a nest, and at the drain: the places a pressure switch fault hides.

4 and 7: the limit. ICP names a dirty filter or a restricted duct system first. Change the filter and open every register: why a furnace overheats and cuts out.

6 and 6+1: ignition. ICP’s first checks are that the gas valve is not turned off and the manual shut-off valve is open. Check both before anything else.

6 again: the flame sensor. ICP says to clean the flame sensor with fine steel wool and recheck its current, nominally 4.0 to 6.0 microamps: how to clean a flame sensor safely.

8 and a solid light: one power reset. ICP says to turn the power off and wait 5 minutes for code 8, and to reset the power to clear a solid light. If the code comes back, stop resetting.

Light off: the power. Check the breaker, the furnace switch, and that the blower door is fully closed on its switch.

When it needs a technician

Check 3

Know when to stop

5, flame with the valve off. A gas valve that may be leaking through. Gas off and a technician.

2, and 3 that survives the vent and drain checks. A pressure switch stuck closed, an obstructed tube, a failed inducer or low inducer voltage.

6 with gas and a clean sensor. ICP’s chart runs a component test and checks the igniter, the gas valve’s 24 volts, the gas pressure and the firing rate.

8 that returns. ICP names a stuck closed gas valve relay on the control, or a miswire or short on the gas valve wire.

10, polarity. Line voltage wired backwards, or on twinned furnaces the transformers out of phase. An electrician’s job.

Stored codes. ICP says the control keeps its last code through a power cut, and a technician can recall it. The chart notes that stored codes are erased after 72 hours.

Questions people ask

What does 3 flashes mean on a Heil furnace?

The pressure switch did not close, or reopened. ICP lists a plugged condensate drain, water in the vent pipe, a restricted vent, wind and the inducer. Check the vent pipes outside and the drain first.

What does 6+1 mean on a Tempstar furnace?

An ignition lockout: the furnace failed to light and prove flame in 4 attempts. ICP says it resets itself after 3 hours. Check the gas is on, then the flame sensor.

What does 4 flashes mean?

A limit or flame rollout switch is open. ICP names a dirty filter or restricted ducts first. If it stays open over 3 minutes, the code becomes 7, a lockout.

Why is the green light on solid?

ICP says the control is locked out and resets itself after one hour. Resetting the power clears it, and a control that locks out again needs replacing.

What does 1+2 mean?

Nothing is wrong. ICP says the blower runs for 90 seconds after the power comes back during a call for heat, and shows 1+2 while it does.

Are Arcoaire and KeepRite codes the same?

They are ICP brands too, and many share the same controls. The service label on your own blower door has the final word for your model.

Sources: ICP (International Comfort Products) Service and Technical Support Manual 440 04 4401 01 for the N9MSE furnace