3 flashes: Draft pressure error
Homeowner checks
The guide gives exactly three suspects: a venting problem, a pressure switch problem, or an inducer problem. That order is a good troubleshooting order, too.
From the table: Red-LED flash codes — integrated furnace control — source: Trane UC Series Installer's Guide 18-CD19D5-10, diagnostic codes (read the document, fetched 2026-10-30). Covers: Trane/American Standard furnaces with the IFC covered by Installers Guide 18-CD19D5-10 (UC/AUH families and relatives). Green LED reads system state; the red LED flashes the faults.
Before the list, rule out the impostor: if you just restored power, the board replays its last four stored faults first. Watch a full minute. A live 3-flash fault repeats as the current code after the replay ends; a stored 3 from last Tuesday is history, not tonight's problem.
For a live one, work Trane's order. Venting: the outdoor terminal — snow, ice, leaves, a wasp nest in October. Switch: the pressure switch and its hose (a look from outside the cabinet is as far as this goes at home). Inducer: is the little exhaust fan actually spinning up when the call starts? A silent inducer with a 3-flash code moves you straight to the technician column.
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 condensate footnote
On condensing Trane furnaces (the 90%+ models with PVC venting), add a fourth suspect the short list implies: the condensate drain. A backed-up drain floods the wrong passages, the pressure switch cannot prove, and the board posts the same 3. If your furnace gurgles or you see water at its base, say so when you call — it reorders the whole diagnosis.
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.