Heil furnace 6 + 3 flashes: Soft lockout — airflow proving circuit opened during the run
Six flashes then three means the maximum recycle count was exceeded and the last failure was the airflow proving circuit opening while the furnace was running.
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 airflow proving circuit confirms the draft inducer is moving air. This code means it proved at ignition and then opened during the run — repeatedly enough to exhaust the recycle allowance. That timing is the useful part. Draft that is adequate at start and lost mid-cycle points at conditions that develop as the furnace operates rather than ones present from the beginning: a condensate trap filling with the water the furnace is producing, ice building at a cold vent termination, or an inducer that weakens as it heats up.
Most common causes
- A condensate drain or trap that backs up during a run and clears between cycles.
- Ice forming progressively at the exhaust termination during a long run.
- An inducer weakening as it warms up.
- A partly restricted vent where draft is marginal rather than absent.
- A pressure switch operating at the edge of its setpoint.
- A sensing hose partly obstructed, so pressure fluctuates during the run.
- High wind at the vent termination disturbing draft intermittently.
How to diagnose it
- Note that the furnace lit and ran before faulting, which rules out the ignition-side causes entirely.
- Note how long it runs before faulting. A consistent interval strongly suggests something accumulating.
- Check the condensate drain for slow drainage rather than complete blockage.
- Check the outside vent termination and note the weather, particularly freezing conditions.
- Listen to the inducer for any change in note during a run.
How to fix it
- Clear a partly blocked condensate drain. This is the most common cause and it is homeowner-reachable.
- Clear the vent termination and keep both pipes free of snow and ice.
- The pressure switch, its hose and the inducer are technician work.
- Have the vent system assessed properly if the restriction is not obvious. Marginal draft is harder to find than absent draft and produces exactly this code.
- Never bypass a pressure switch. An intermittent draft is still a draft problem.
Notes
Run-mode faults are harder to reproduce on a service visit than start-up faults, because the technician has to wait out a full cycle. Knowing the elapsed time in advance saves that wait.
Related
- 6 + 2 flashes — Soft lockout — flame lost during run, cycling pressure switch, or blocked condensate
- 3 flashes — Low pressure switch still open 30 seconds after the inducer started
All codes for this manufacturer
Heil, Tempstar and Comfortmaker furnace codes — every code, with what differs between models.
What the fix costs
Draft that proves at ignition and fails mid-run points at a short list. Cheapest: ice or an obstruction building at the vent termination, cleared by hand or within a service call. Middle: a condensate path that fills as the furnace produces water — a clearing-and-correction job in the repair guide's $75–150 diagnostic-plus-labor territory. The real part: an inducer that starts strong and weakens as it heats, at $300–900 installed, the widest range on the sheet because brand availability sets the price. The pressure switch itself, $150–400, is the least likely culprit here precisely because it proved fine at the start of every cycle.
Do it yourself or call someone?
Your two contributions: the termination and the clock. Looking over the outdoor intake and exhaust for progressive icing — especially in hard cold, where the furnace's own moist exhaust builds the ice — is safe and directly relevant. So is timing the failures: a furnace that always drops out twenty minutes into a run is describing a filling trap or a heating motor, and that number shortens the diagnosis. Inside the cabinet is the technician's side of the line, and running the furnace on repeated resets against a failing draft proof means running it against the safeguard that keeps flue gas moving outdoors.
Related codes
Allied's 9 flashes covers the same opened-during-demand case alongside never-closing; the site-wide picture of draft-proving faults is on inducer motor codes and pressure switch codes.
What this looks like
From the outside, this usually shows up as the vents blowing cold air on a heat call — that walkthrough covers what to check in order, and when it stops being a homeowner job.