Armstrong furnace 9 flashes: Pressure switch failed to close, or opened during a heat demand
Nine flashes means the pressure switch never closed, or closed and then opened during the heat demand. It is the most common fault on any condensing furnace.
Applies to: Allied Air Enterprises uses the same -02 integrated control across its A80UH1E and 80G1UHE gas furnaces, sold under the Armstrong Air, AirEase, Concord, and Ducane names.
What it means
The pressure switch proves the draft inducer is moving enough air to carry combustion products safely out of the house before the gas valve opens. This code covers both failure directions in time: never closing at all, or closing and dropping out mid-cycle. That distinction is worth drawing yourself. Never closing points at something present from the start — a blocked vent, a full condensate trap, a disconnected hose. Dropping out mid-run points at something that develops during the cycle, such as a trap filling with the water the furnace is producing or ice building at a cold termination.
Most common causes
- 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.
- High wind at the termination on exposed installations.
How to diagnose it
- Look at the outside vent termination first. It is the highest-yield check on the list and needs no tools.
- Note whether the furnace lights at all before faulting. Never lighting versus faulting mid-run separates the causes.
- Listen for the inducer. If it runs and nothing follows, the switch is not seeing the draft.
- Check the condensate drain and trap for standing water.
- Note the weather. This appearing during the first hard freeze or a snowstorm points strongly at the termination.
How to fix it
- Clear the vent termination if blocked, and keep both pipes clear of snow, ice and vegetation.
- Clear a blocked condensate drain — reasonable homeowner work on most installations.
- The pressure switch, its hose, the inducer and the vent system inside the cabinet are technician work.
- If vent sizing is wrong for the furnace, have it corrected rather than worked around. A furnace that only just proves draft will fault intermittently for years.
- Never bypass the pressure switch. A blocked flue with a firing burner is how carbon monoxide enters a house.
Notes
If this appears on the coldest night and clears when milder, suspect ice at the vent termination. High-efficiency furnaces produce condensate at the exhaust and it can freeze around the pipe, progressively closing it.
Related
- 10 flashes — Watchguard — pressure switch opened five times during one heat demand
- 11 flashes — Pressure switch stuck closed before the inducer started
All codes for this manufacturer
Armstrong Air, AirEase, Concord and Ducane 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 9 flashes means on every brand puts them side by side.
What the fix costs
The furnace repair cost guide lists a pressure switch at $150–400, with the note that matters more than the range: check for a blocked flue first. On this code the switch is frequently reporting the truth — a vent obstruction, a full condensate trap, a slipped sensing hose — and the honest bill is a service call plus a clearing rather than a part. The pricier branch is a genuinely weakening inducer, at $300–900 with brand availability driving the spread. Both directions this code covers, never-closed and dropped-out-mid-run, price the same; they just point the technician at different evidence.
Do it yourself or call someone?
One safe check earns its keep: walk outside and look at the intake and exhaust terminations. Snow drifted over a sidewall vent, ice, leaves, or a nest are common causes of exactly this code and are visible without opening anything. What is not homeowner work: pulling sensing hoses, poking at the condensate trap, or — ever — jumpering the switch to make the furnace run. This is the interlock that proves combustion gases are leaving the house; defeating it converts a no-heat problem into a venting-into-the-house problem.
The same fault on other control families
A pressure switch that will not close is the most widely coded fault in furnaces — 47 codes across 12 families, collected on pressure switch codes. Direct equivalents include Carrier's code 31, Goodman's 3 flashes, and Trane's 3 flashes; the ICP SmartValve version, 3 flashes, documents the retry timing most brands leave unstated.
What this looks like
From the outside, a pressure switch that won’t stay closed usually shows up as the furnace starts and stops in short bursts, proving draft and then losing it mid-demand — that walkthrough covers what to check in order.