2+3
Low pressure switch failed open
This code typically points to
- Vent restriction
- Weak inducer
- Tubing leak or water in tubing
- Condensate trap blockage
Next checks to confirm
- Verify inducer energizes
- Verify pressure switch closes within allowed proving period
- Per ML180UH public sequence, 2.5 minutes to prove then pressure-switch watchguard starts with 5-minute reset period
- Measure operating pressure in inches w.c. at the switch
Escalation
Heat sequence aborted; watchguard engages if proving repeatedly missed
Reset behavior
Auto-retry after reset period; manual thermostat interruption also resets sequence path in some branches.
Urgent
Fix now