Heil furnace 6 + 4 flashes: Soft lockout — limit circuit opened during the run

Six flashes then four means the recycle count was exhausted and the last failure was the limit circuit opening while the furnace was running. That is overheating, repeatedly.

Applies to: International Comfort Products *9MPT and *9MPV gas furnaces using the Honeywell SV9541Q SmartValve, sold as Heil, Tempstar and Comfortmaker.

What it means

The limit circuit opening mid-run means the furnace got too hot while operating. Unlike a limit that is open before ignition, this one proves the furnace heats fine and cannot sustain it — which points squarely at airflow. Air is not carrying heat away fast enough, the heat exchanger temperature climbs through the cycle, and the limit opens. That it has happened enough times to exhaust the recycle allowance means it is not a one-off. This is the pattern that damages heat exchangers over time, so it is worth resolving rather than resetting.

Most common causes

A clogged air filter, which is by a wide margin the most common cause.
Closed, blocked or covered supply registers, or too many closed at once.
A blocked return air grille.
Undersized or restrictive ductwork, or a duct that has come apart or collapsed.
A failing blower motor, or a blower wheel packed with dust so it cannot move rated air.
Blower speed set too low for the furnace's output.
A dirty evaporator coil above the furnace, which is a frequent hidden restriction.

How to diagnose it

Check the air filter first. Grey, matted or opaque to light means you have very likely found it.
Note that the furnace runs before faulting — that is the signature of an airflow problem rather than a switch failure.
Note how long it runs before tripping. Heating normally at first and tripping after twenty minutes is the classic marginal-airflow pattern.
Walk the house and count closed or blocked supply registers, and check the return grille.
Note whether the blower sounds weaker than it used to.

How to fix it

Replace the air filter with the correct size and type, and check it monthly during the heating season. A filter far more restrictive than the system was designed for causes this as reliably as a dirty one.
Open all supply registers and clear the return grille.
Blower motor work, blower wheel cleaning, evaporator coil cleaning, duct repair and speed changes are technician tasks.
Do not keep resetting it. Repeated overheating is what cracks heat exchangers, and a cracked heat exchanger is a carbon monoxide risk rather than a comfort problem.

Notes

Closing registers in unused rooms feels like an obvious saving and is a common cause of this. A forced air system needs its designed airflow through the furnace regardless of which rooms want heat, and restricting it moves the problem to the heat exchanger.

All codes for this manufacturer

Heil, Tempstar and Comfortmaker furnace 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

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.