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.
Related
- 4 flashes — Limit switch string is open
- 7 flashes — Soft lockout — limit trips taking longer than two minutes to reset
All codes for this manufacturer
Heil, Tempstar and Comfortmaker furnace codes — every code, with what differs between models.
What the fix costs
Almost every ending to this code is an airflow bill. The switch itself sits in the repair guide's $150–350 limit band, but a limit that opens only mid-run is nearly always reporting true overheating, so the spend goes to the cause: free if it is a clogged filter or closed registers, $150–350 if a PSC blower's capacitor is letting airflow sag — the blower cost guide calls the capacitor check the first thing an honest technician does — and $400–1,500 if the blower itself is failing. The cost of not fixing it compounds quietly: each overheat cycle stresses the heat exchanger, the one part whose failure re-prices everything at $1,000–2,500+.
Do it yourself or call someone?
Start with the two airflow checks that need no tools: a fresh filter and every supply register open. If the lockouts stop, done. If they continue, resist the reset habit this code specifically invites — the furnace heats fine for a while each time, so clearing it feels productive while the exchanger takes another overheat. A blower that has slowed (weak airflow at the registers is the tell you can feel) is line-voltage diagnosis, and capacitors hold their charge after power-off; that half belongs to the visit. Mention whether airflow felt weak when you book — it routes the appointment.
Related codes
On this same valve, 7 flashes is the escalated version — limit trips taking over two minutes to reset. On the Goodman GMEC96-family table, the blower-failure flavour of a limit trip gets its own code, 11 red. Cross-brand: limit switch codes.
What this looks like
From the outside, this usually shows up as the furnace heats then trips out on high limit — that walkthrough covers what to check in order, and when it stops being a homeowner job.