Flame Without Gas Valve Energized
Flame detected when gas valve is not commanded on.
Probable Causes
- Sticking gas valve
- Short in flame sensor circuit
Diagnostic Steps
- SAFETY: Shut off gas immediately
- Test gas valve for leak-through
- Check flame sensor wiring
Pressure Switch Stuck Closed
Pressure switch reading closed before inducer starts.
Probable Causes
- Stuck pressure switch contacts
- Inducer running inadvertently
Diagnostic Steps
- Test pressure switch with hose disconnected
- Check inducer operation at board
Pressure Switch Stuck Open
Pressure switch failed to close after inducer starts.
Probable Causes
- Blocked vent
- Bad pressure hose
- Weak inducer motor
- Condensate blockage
Diagnostic Steps
- Check flue and intake
- Inspect hose to switch
- Test inducer amps
- Clear condensate drain
Open High Limit Device
High limit switch has tripped due to overheating.
Probable Causes
- Dirty filter
- Blocked returns
- Bad blower motor
- Oversized gas orifice
Diagnostic Steps
- Replace air filter
- Check all returns are open
- Test blower operation
- Check gas pressure
Flame Rollout Switch Open
Flame rollout safety switch has tripped.
Probable Causes
- Heat exchanger crack
- Blocked heat exchanger
- Gas overpressure
Diagnostic Steps
- SAFETY: Check for CO
- Inspect heat exchanger for cracks
- Test CO levels
- Check gas pressure
Ignition Failure
Furnace failed to ignite after inducer proves.
Probable Causes
- Weak or broken igniter
- No gas supply
- Dirty flame sensor
Diagnostic Steps
- Verify gas supply is on
- Check igniter resistance (40-200 ohms)
- Clean flame sensor with fine steel wool
- Check ground wire connection
Ignition Lockout - Max Retries
Furnace locked out after maximum ignition attempts.
Probable Causes
- Repeated ignition failures — see code 6
Diagnostic Steps
- Power cycle to clear lockout
- Address underlying issue from 6-flash code