Heil furnace 3 flashes: Low pressure switch still open 30 seconds after the inducer started

Three flashes means the inducer ran and the low pressure switch had still not closed after 30 seconds. The control then waits five minutes with the inducer off before trying again.

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

What it means

ICP documents this with unusual precision: the low pressure switch circuit was still sensed as open 30 seconds after the inducer was energised, and the system enters a five-minute delay with the inducer off before initiating a new ignition sequence. That timing explains a symptom people often misread — a furnace that runs its inducer, stops, sits silent for five minutes, then tries again. That is not the furnace being temperamental, it is the documented retry cycle. ICP also notes a specific and easily missed cause: the SV9541Q's own on/off switch left in the off position during a call for heat generates this code.

Most common causes

• The SmartValve's on/off switch in the off position during a call for heat, which ICP names explicitly.
• A blocked exhaust or intake pipe at the outside termination — snow, ice, leaves, insect or rodent nests.
• A condensate drain or trap that is full or blocked.
• The pressure switch sensing hose disconnected, cracked, kinked or holding water.
• A weak or seized draft inducer.
• A failed pressure switch that no longer closes at its rated pressure.
• Vent piping too long, with too many elbows, or the wrong diameter for the furnace.

How to diagnose it

1. Check the gas valve's own on/off switch first. ICP lists it as a cause of this exact code and it costs nothing to look.
2. Look at the outside vent termination. It is the highest-yield check on the list and needs no tools.
3. Note the five-minute rhythm. Inducer runs, stops, five minutes of silence, tries again — that pattern confirms this code rather than an intermittent fault.
4. Check the condensate drain and trap for standing water.
5. Note the weather. This appearing during the first hard freeze or a snowstorm points strongly at the termination.

How to fix it

1. If the SmartValve's on/off switch is off, that is the fault. Turn it on — but establish why it was off, since someone turned it off for a reason.
2. Clear the vent termination if blocked, and keep both pipes clear of snow, ice and vegetation.
3. Clear a blocked condensate drain — reasonable homeowner work on most installations.
4. The pressure switch, its hose, the inducer and the vent system inside the cabinet are technician work.
5. Never bypass the pressure switch. A blocked flue with a firing burner is how carbon monoxide enters a house.

Notes

The documented 30-second window and five-minute delay make this one of the more diagnosable furnace faults. If the timing you observe does not match, you are probably looking at a different code.

Related

• 2 flashes — Low pressure switch closed when it should be open
• 9 flashes — High pressure switch open when it should be closed

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 3 flashes means on every brand puts them side by side.

Sources

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