4+3
Low-heat pressure switch open while high-heat pressure switch is closed
This code typically points to
- Plugged condensate drain
- Low inlet gas pressure if LGPS used
- Improper pressure-switch wiring
- Water in vent piping, possibly sagging pipe
- Disconnected or obstructed pressure tubing
- Stuck open low-heat pressure switch
Next checks to confirm
- Check condensate drainage first
- Verify inlet gas pressure if low-gas-pressure switch is used
- Inspect pressure-switch wiring
- Inspect vent piping for water/sagging
- Inspect pressure tubing for disconnection or obstruction
- Test low-heat pressure switch for open/stuck-open condition
Escalation
Escalate to control/pressure-proving circuit diagnosis only after condensate, venting, tubing, wiring, gas-pressure, and low-heat switch condition are verified.
Reset behavior
No distinct timed auto-reset; treat as active status that should clear when contradictory pressure-switch condition is corrected.
Urgent
Fix now