Trane furnace 3 flashes: Draft pressure error
Three flashes means a draft pressure error. Trane names three possible areas: a venting problem, a pressure switch problem, or an inducer problem.
Applies to: Trane and American Standard upflow, downflow and horizontal gas-fired direct vent condensing furnaces covered by Installer's Guide 18-CD29D1-14-EN, using the integrated furnace control described there.
What it means
The furnace must prove it is moving combustion air before gas is released, and this code means that proof failed. Trane's documentation is helpfully structured here — rather than naming a single component it lists three areas, and that ordering reflects how the fault usually breaks down. Venting comes first because it is the most common and the most likely to be caused by something outside the furnace entirely: a vent termination blocked by snow, ice, leaves, or a nest. The pressure switch and the inducer are the components in the path, and both are inside the cabinet.
Most common causes
- A blocked exhaust or intake pipe at the outside termination — snow, ice, leaves, insect or rodent nests.
- Vent piping too long, with too many elbows, or the wrong diameter for the furnace.
- A condensate drain or trap that is full or blocked.
- The pressure switch sensing hose disconnected, cracked, kinked, or holding water.
- A failed pressure switch that no longer closes at its rated pressure.
- A weak or seized inducer motor, or a loose inducer wheel.
- High wind at the vent termination on exposed installations.
How to diagnose it
- Look at the outside vent termination first. It is the highest-yield check and needs no tools.
- Listen for the inducer. If it does not run at all, the fault is upstream; if it runs and nothing follows, the pressure 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 at the termination.
- Note whether anything has recently been built, stored, or planted near the outside vent.
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 vent work inside the cabinet are technician tasks.
- If vent sizing is wrong for the furnace, have it corrected. A furnace that only just proves draft will fault intermittently for years.
- Never bypass the pressure switch. It is what prevents the furnace firing into a blocked flue.
Notes
Trane listing three areas rather than one component is worth taking at face value. Replacing a pressure switch on a furnace whose real problem is a restricted vent fixes nothing and costs a part.