Heil furnace 2 flashes: Low pressure switch closed when it should be open
Two flashes means the low pressure switch was found closed before the inducer ran. It must sit open at rest, and finding it closed means the draft-proving interlock cannot be trusted.
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 pressure switch proves the draft inducer is genuinely moving air before gas is released, and it does that by sensing suction. At rest, with the inducer off, it must be open. The control verifies this before every cycle, because a switch stuck closed would prove a draft that does not exist — which would let the furnace fire into a flue that might be blocked. This is the interlock working as designed, and the code points at a specific component rather than a general area.
Most common causes
- Pressure switch contacts stuck or welded closed.
- The sensing hose pinched or kinked so that it traps pressure.
- Water in the sensing hose or the pressure tap holding the switch closed.
- A short in the pressure switch wiring, which is electrically indistinguishable from closed contacts.
- The wrong pressure switch fitted at a previous repair. These are rated per model and are not interchangeable.
- The low and high stage switches swapped during service.
How to diagnose it
- Note whether the inducer starts at all. On this fault the control usually stops before that step.
- Note whether the furnace was serviced recently. A hose or switch disturbed during other work is a common trigger.
- Check the condensate drain for backing up, since water reaching a sensing hose is a frequent cause on condensing furnaces.
- Distinguish this from the three-flash code carefully. They are opposite halves of the same test and need different repairs.
How to fix it
- Clearing a blocked condensate drain is reasonable homeowner work and addresses one of the causes.
- The pressure switches, hoses and wiring are technician work. They prove safe venting, and a furnace running with one defeated can vent combustion products indoors.
- Never jumper a pressure switch to get heat. It is the only thing between a blocked flue and a firing burner.
- Replace a failed switch with the exact part for the model, since the setpoint is model-specific.
Notes
On the other ICP diagnostic system — the furnace control board — two flashes means something different again. Confirm which light you are reading before acting.
Related
- 3 flashes — Low pressure switch still open after the inducer started
- 8 flashes — High pressure switch closed when it should be open
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 2 flashes means on every brand puts them side by side.