The short answer
Three red flashes on a Trane furnace in Northern Virginia is a draft pressure fault. The pressure switch didn’t prove that the vent is clear and the inducer fan is pulling air, so the furnace won’t light. On a furnace with a number display, the same fault reads E3.1, E3.2 or E3.3, and a blocked drain shows E09. Trane’s own guide says that in most cases the switch itself isn’t the problem. A blocked pipe, a frozen drain or a tired inducer usually is.
What the pressure switch does
Before the furnace lights, a small fan called the inducer starts and pulls air through the burners and out the vent. The pressure switch is a little round part with a rubber hose running to it. When it feels that suction, it closes and tells the board it’s safe to light. No suction, no fire. That’s the point: it keeps a furnace with a blocked vent from filling the house with exhaust.
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E3.1, E3.2, E3.3 and E09
Swipe to see all columns →
Swipe to see all columns →
| Code | What happened | Usual cause |
|---|---|---|
| E3.1 | First-stage switch closed when it should be open | A stuck switch |
| E3.2 | First-stage switch didn’t close | Vent, drain, hose or inducer (the common one) |
| E3.3 | Second-stage switch closed when it should be open | A stuck switch |
| E09 | Inducer limit or condensate switch open | A blocked or frozen drain |
Trane’s S9X installer guide says it plainly for E3.2: “In most cases, the pressure switch is not the problem.” On older single-light and two-stage boards it’s just 3 flashes, which Trane’s single-stage guide calls a draft pressure error: the vent, the pressure switch or the inducer.
The causes, most common first
Snow, ice or leaves in the pipes outside
A 90% or higher furnace vents through two white plastic pipes on an outside wall or the roof. Drifting snow, ice on the pipe ends, leaves and the odd nest block them. Look there first. Clearing snow away from the pipe ends is safe to do yourself.
A frozen or clogged condensate drain
High-efficiency furnaces make water, and it runs out through a drain trap and a small hose. When the trap clogs with sludge, or the line freezes where it runs through a cold garage or crawlspace, water backs up into the inducer housing and the switch can’t close. Trane’s guide says a blocked drain causes E09 and, in extreme cases, E04. Furnace condensate drain clogged or frozen walks through it. Clearing a drain runs $175 to $420.
Wind
Trane’s two-stage guide says wind over 40 mph can blow the pressure switch open. The board ramps the inducer to full speed for about a minute, rests about 30 seconds, and tries again. If the code only shows up on windy nights, that’s your likely reason, and it’s worth a tech looking at where the vent ends.
A cracked hose or water in it
The rubber hose from the inducer to the switch cracks with age or fills with water. On the S9X, the condensate pressure switch hose has to be trimmed right at install, and Trane says an untrimmed one can cause E09.
A weak inducer motor
The inducer’s bearing wears and the fan slows down. You’ll often hear it first: a whine or a rattle at startup. A tech measures the suction before replacing anything.
The switch itself
Sometimes it really is the switch. A tech proves it with a meter before buying one. Our Lennox pressure switch guide covers the same part on another brand.
Never jumper the switch
You’ll find videos showing how to bypass a pressure switch with a wire. Don’t. That switch is what keeps exhaust out of the house when the vent is blocked. Bypass it, and the furnace can run with nowhere for carbon monoxide to go. If your carbon monoxide alarm is going off, get everyone out first and call from outside.
What it costs
A diagnostic visit is $99, and you get the problem and the price in writing before any work starts. Clearing a drain runs $175 to $420. A pressure switch or an inducer motor gets priced at the furnace once the tech has tested it. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. The full list of Trane furnace error codes covers every other blink.
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Questions people ask
What does 3 flashes mean on a Trane furnace?
A draft pressure fault. The pressure switch didn’t prove the vent is clear, so the furnace won’t light. Check the white plastic pipes outside for snow, ice or leaves, then the condensate drain.
What is E3.2 on a Trane furnace?
The first-stage pressure switch didn’t close. Trane says in most cases the switch isn’t the problem: look at the vent, the drain, the hose to the switch and the inducer motor first.
What is E09 on a Trane furnace?
The inducer limit or the condensate switch opened. A blocked or frozen condensate drain is the usual cause.
Can wind cause a Trane furnace pressure switch error?
Yes. Trane says gusts over 40 mph can open the pressure switch. The board speeds the inducer up, rests, and tries again.
Should I replace the pressure switch on my Trane furnace?
Not until somebody tests it. Most of the time it’s the vent, the drain or the inducer, and a new switch won’t fix any of those. It’s the part people buy first and the one that’s least often bad.
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