3 red flashes: Pressure switch stuck open
Homeowner checks
The manual lists the full suspect line-up: faulty combustion air blower, blocked vent pipe, blocked air intake pipe, blocked condensate drain, faulty pressure switch hose, or a faulty pressure switch.
From the table: Red-flash codes — York user-manual table — source: York PC9 / GF9S User Manuals (Unitary Products Group), Furnace Control Diagnostics (read the document, fetched 2026-10-30). Covers: York furnaces covered by the PC9 / GF9S / G8C user and service manuals (Unitary Products Group). Flashes are 1/4 second on, 1/4 second off, with a 2-second pause between groups.
York's manual, nearly verbatim in structure: "This could be caused by a faulty combustion air blower, blocked vent pipe, blocked air intake pipe, blocked condensate drain, faulty pressure switch hose or a faulty pressure switch." Six suspects, and they split cleanly by who can check them. The vent pipe, the intake pipe, and the condensate drain announce themselves from outside — snow against the terminal, an intake full of leaves, a drain running slow or frozen. The blower, the hose, and the switch live behind the cabinet door.
One observation is free and decisive: when the thermostat calls, do you hear the inducer (the small exhaust fan) start? If it never spins, the switch has nothing to prove — the fault is at the top of York's list, and it is a motor call.
Homeowner checks — the safe list:
- Go outside and look at the vent and intake pipes. Snow, ice, leaves, a nest, or a mulch pile against the terminal can cause exactly this code.
- On a condensing (90%+) furnace, check the condensate drain: a backed-up drain or frozen drain line trips the same switch.
- Air filter: a starved furnace is a furnace that overheats and behaves badly in several code families.
- Make sure the blower door is fully seated after you look at the light.
The door-chart warning, from a York dealer’s own notes
One of the dealer knowledge bases that publishes York's tables adds a warning worth repeating: do not go by the short fault descriptions printed on some blower doors — for code 3 the door may say "vent or termination problem," which the dealer notes is only one, and one of the least common, causes. The full manual list above is the better map. It is also, quietly, the whole philosophy of this library: the short version of a code is where bad diagnoses come from.
The two rules that outrank everything on this page.
1. Smell gas? Stop reading. Leave the house and call your gas utility or 911 from outside. No code is worth diagnosing through a gas smell.
2. Never bypass a safety switch — no jumping a pressure switch, no taping a limit, no propping a rollout. Those switches are the only thing between a furnace fault and a fire or carbon-monoxide problem. This library explains codes; it does not teach repairs inside the cabinet.
One honesty note before the tables: codes change between control boards, sometimes inside the same brand and the same decade. Every table below names the board and the manual it was transcribed from. The chart glued inside your own blower door is the final word for your furnace — if it disagrees with this page, believe the door.