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 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 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.

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.

This code is not a homeowner repair. Faults in this group involve the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. Do not reset it repeatedly to keep the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on.

Sources

Allied Air Enterprises — A80UH1E and 80G1UHE Installation Instructions 507330-03, Table 7: Diagnostic Codes for -02 Control

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.