Trane furnace E3.4: Open pressure switch, second stage
E3.4 means the second stage pressure switch did not close. The furnace can usually still run on first stage, which is why this fault often shows up as poor heating rather than no heating.
Applies to: Trane and American Standard S9V2 two-stage condensing variable speed gas furnaces with a seven-segment integrated furnace control display, per Installer's Guide 18-CE01D1-1E-EN.
What it means
A two-stage furnace has a low fire and a high fire, and the higher firing rate needs its own proof of draft — a second pressure switch set for the greater airflow the second stage inducer speed produces. E3.4 says that second switch never closed. The practical effect is different from a first stage fault: the furnace is not dead. It lights and runs on low fire and simply never reaches full output. In cold weather that presents as a house that will not get up to temperature and a furnace that runs constantly, which most people do not recognise as a fault code at all. The furnace is working, at half its capacity.
Most common causes
- A partially restricted vent or intake — enough draft for first stage, not enough for second.
- Condensate in the second stage pressure switch hose or the collector box.
- The inducer not reaching its second stage speed.
- A cracked or disconnected hose on the second stage switch specifically.
- A failed second stage pressure switch.
How to diagnose it
- Note whether the house struggles to reach temperature in cold weather while the furnace runs constantly. That is this fault's signature.
- Check the vent and intake terminations for partial obstruction as well as complete blockage.
- Check the Last 6 Faults menu — this fault can be present for a long time without anyone noticing.
- Note whether the problem is seasonal, since second stage is called far less in mild weather.
How to fix it
- Clear any obstruction at the terminations.
- The rest is technician work, and it is worth doing before deep winter rather than during it.
- A furnace stuck on first stage will not fail dangerously, but it will fail to keep up when it matters most.
Notes
This is the fault most likely to go unreported for a whole season, because the furnace still produces heat. Checking the fault history at a service visit is how it gets found.
Related
- E3.2 — Open pressure switch, first stage
- Which Trane code system you have — Trane furnaces use either flash codes or a seven-segment display, and they share no numbering
All codes for this manufacturer
Trane and American Standard furnace flash codes — every code, with what differs between models.
What the fix costs
A furnace that still heats changes the economics of this code more than the parts do. The likely repairs are the same as any pressure switch fault — the repair guide's $150–400 switch, or clearing a partial vent restriction for nothing — but E3.4's real cost lever is timing. Handled at a scheduled autumn visit, it is standard-rate work. Discovered in a January cold snap, when second stage is finally demanded and the house will not reach temperature, the same job carries the guide's $100–200 emergency premium and a queue. A half-capacity furnace is a repair on your schedule; few codes offer that.
DIY or call a pro
Two things are homeowner territory. The terminations: E3.4's signature cause is a partial restriction — enough draft for low fire, not enough for high — so look for the half-blocked intake, the drifted snow, the vegetation that grew over summer, not just total blockage. And the symptom log: a house that lags in cold weather while the furnace runs continuously is this fault's fingerprint, and it is diagnosable from the couch. What is not DIY is anything involving the switch, its hose, or inducer speeds inside the cabinet. Book the visit before deep winter; the furnace is running on the one stage that does not need this switch.
The same fault on other brands
In the industry-wide stuck-open family on the pressure switch hub — three red on the GMEC96-family board, two blinks on Rheem's *RC controls, Lennox 223 — the equivalents all stop the furnace outright, because the failed switch is the only one proving draft. E3.4 is the family's mildest member: the first stage switch still proves low-fire draft, so the furnace degrades instead of dying. Same fault class, uniquely soft landing — and the reason this one hides in fault histories for whole seasons.
What this looks like
From the outside, this usually shows up as the furnace starts and stops in short bursts — that walkthrough covers what to check in order, and when it stops being a homeowner job.