Field pattern: 90%+ furnace pressure switch code that comes back after a new switch (condensate trap and hose first)
The switch is usually reporting correctly: water backed up in the trap, the collector box or the switch tubing is blocking the vacuum it needs to sense. The wrong turn is replacing the switch (sometimes twice) without clearing the trap and tubing; a partially clogged trap drains while the furnace is locked out, so it runs fine on the first test and fails after the tech leaves.
Cross-checked ~60 min moderateBrands: All
Before you start
- Turn off power before removing the trap or tubing
- Never jumper the pressure switch to run the furnace; it is a draft-proving safety
- Vent and intake pipe repair, inducer-to-vent connections and anything involving combustion gases are BLOCKED: refer to a licensed technician
- Condensate is acidic; wear gloves and eye protection
Diagnostic steps
Unlock all 8 steps with expected values
Full library + AI assistant + the OnCall Tech app for iPhone & Android. $34.90, one time.
Get lifetime accessOnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.
