Trane furnace E3.1: Shorted pressure switch, first stage
E3.1 means the first stage pressure switch reads closed when it should be open — before the inducer starts. A switch that is always closed cannot prove anything, so the furnace stops.
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
Trane's sequence of operation proves both pressure switches open before the inducer runs, by sending 24 volts out the HLO terminal through the limit switches and watching the PS1 and PS2 inputs. With the inducer off there is no suction, so a working switch must sit open. Reading it closed means the switch is stuck, welded, shorted in its wiring, or bypassed — and in every one of those cases the interlock that proves the flue is clear has been defeated rather than merely failed. That is why this is a stop rather than a warning. The switch would read closed with a completely blocked vent just as readily.
Most common causes
- A failed pressure switch with stuck or welded contacts.
- A short in the pressure switch wiring, electrically identical to a closed switch.
- A switch jumpered during earlier service and never restored.
- A pressure switch of the wrong specification fitted in a previous repair.
- Moisture or corrosion in the switch or its connector bridging the contacts.
How to diagnose it
- Note whether the furnace has been serviced recently. A jumper left across a pressure switch is a real and dangerous cause.
- Note whether the inducer starts at all — on this fault the control should stop before it does.
- Trane's note that 24 volts will not be present at either pressure switch if a thermal limit is open matters here: an open limit can mask or confuse this reading.
- Nothing here is a homeowner check.
How to fix it
- Technician work. This is the interlock proving the flue is clear.
- The wiring should be checked for a short before the switch is condemned.
- If a jumper is found it must come out, and the reason it was fitted needs to be established.
- A replacement switch must match the specification for the model, not merely fit the fitting.
Notes
E3.1 and E3.3 are the shorted conditions for first and second stage; E3.2 and E3.4 are the open ones. Once that symmetry is clear the whole E3 family reads at a glance.
Related
All codes for this manufacturer
Trane and American Standard furnace flash codes — every code, with what differs between models.