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 uses two unrelated code systems
All codes for this manufacturer
Trane and American Standard furnace flash codes — every code, with what differs between models.