No Previous Error
System has no stored fault codes. Normal operation.
Diagnostic Steps
- No action needed
System Lockout - Retries Exhausted
Furnace has failed to ignite after maximum retries and is in safety lockout.
Probable Causes
- Repeated ignition failure
- Gas supply interrupted
- Flame sensor contaminated
- Igniter weak or failed
Diagnostic Steps
- Power cycle to reset lockout
- Verify gas supply and manual valve
- Clean flame sensor with fine steel wool
- Test igniter resistance
- Check gas valve operation
Pressure Switch Error
Pressure switch failed to close or opened during operation.
Probable Causes
- Blocked flue or intake
- Bad pressure switch
- Inducer motor failure
- Pressure hose disconnected or cracked
Diagnostic Steps
- Check flue and intake pipes for obstructions
- Inspect pressure hose connections
- Test pressure switch operation
- Measure inducer motor amps
Open High Limit Device
High temperature limit switch has opened due to overheating.
Probable Causes
- Dirty air filter
- Blocked return air
- Slow or failed blower motor
- Oversized gas input
Diagnostic Steps
- Replace air filter
- Check all vents and returns are open
- Test blower motor operation
- Verify gas pressure
Flame Sensed Without Gas Valve Energized
Flame detected when gas valve is not commanded on — potential gas leak.
Probable Causes
- Gas valve leaking internally
- Short in flame sensing circuit
- Board malfunction
Diagnostic Steps
- SAFETY: Close manual gas valve immediately
- Test gas valve for leak-through
- Inspect flame sensor wiring
- Replace gas valve if confirmed leaking
Power or Low Voltage Interrupt
Power disruption or 24V low voltage condition detected.
Probable Causes
- Power flicker
- Loose wiring
- Weak transformer
- Thermostat wire issue
Diagnostic Steps
- Check transformer output voltage
- Inspect all wire connections
- Test for thermostat wire damage or short
Gas Valve Circuit Error
Problem with the gas valve electrical circuit.
Probable Causes
- Bad gas valve coil
- Wiring fault to gas valve
- Board output failure
Diagnostic Steps
- Test gas valve coil resistance
- Check wiring between board and gas valve
- Test board gas valve output voltage
Check Polarity / Ground Fault
Line voltage polarity is reversed or ground is missing.
Probable Causes
- Hot and neutral reversed
- No ground wire
- Floating neutral
Diagnostic Steps
- Check outlet polarity with meter
- Verify ground connection
- Correct wiring at panel