Armstrong furnace 11 flashes: Pressure switch stuck closed before the inducer started
Eleven flashes means the pressure switch was already closed before the combustion air inducer was activated. It must sit open at rest, and finding it closed means the interlock cannot be trusted.
Applies to: Allied Air Enterprises A80UH1E and 80G1UHE gas furnaces with the -02 integrated control, sold as Armstrong Air, AirEase, Concord and Ducane.
What it means
The pressure switch proves the inducer is genuinely moving air, and it does that by sensing suction. At rest, with the inducer off, it must be open. The control checks this before activating the inducer, 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. Allied Air's wording is precise about the timing: stuck closed prior to activation of the combustion air inducer, so this check happens before anything else in the sequence.
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.
How to diagnose it
- Note whether the inducer starts at all. On this fault the control stops before that step by design.
- 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 the sensing hose is a frequent cause.
- Distinguish this from the nine-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 switch, its hose and its wiring are technician work. The switch proves safe venting.
- 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
Nine and eleven flashes are the two failure directions of one test — one says the switch will not close, the other says it will not open. Counting accurately matters more here than almost anywhere else in the set.
Related
- 9 flashes — Pressure switch failed to close or opened during heat demand
- 10 flashes — Watchguard — pressure switch opened five times
All codes for this manufacturer
Armstrong Air, AirEase, Concord and Ducane furnace codes — every code, with what differs between models.