No Power to Control Board
The furnace control board has no power.
Probable Causes
- No 120V line voltage
- Blown fuse on board
- Tripped breaker
- Bad transformer
- Open door switch
Diagnostic Steps
- Check breaker panel
- Inspect board fuse (3A or 5A)
- Verify door switch is closed
- Test 120V at board
- Test 24V transformer output
System Lockout - Ignition Failure
Furnace locked out after repeated ignition failures.
Probable Causes
- No gas supply
- Failed igniter
- Dirty flame sensor
- Gas valve fault
Diagnostic Steps
- Power cycle to clear lockout
- Check gas supply
- Test igniter resistance (40-200 ohms)
- Clean flame sensor
- Test gas valve operation
Normal Operation - No Call
System is powered and in standby. No call for heat.
Diagnostic Steps
- No action needed — normal standby
Pressure Switch Stuck Open
Pressure switch failed to close with inducer running.
Probable Causes
- Blocked vent or intake
- Cracked pressure hose
- Inducer motor failure
- Condensate drain blocked
Diagnostic Steps
- Check flue termination for obstruction
- Inspect hose to pressure switch
- Test inducer motor amps
- Clear condensate drain
Pressure Switch Stuck Closed
Pressure switch is reading closed before inducer starts.
Probable Causes
- Stuck pressure switch contacts
- Inducer running inadvertently
Diagnostic Steps
- Disconnect pressure switch hose — switch should open
- Test switch with multimeter
- Check for stuck inducer relay on board
Open High Limit Switch
High temperature limit has tripped.
Probable Causes
- Dirty air filter
- Restricted airflow
- Blower motor fault
Diagnostic Steps
- Replace air filter
- Check all vents and returns are open
- Test blower motor operation
- Verify ductwork not collapsed
Flame Sensed Without Call for Heat
Flame detected when system should not be heating.
Probable Causes
- Leaking gas valve
- Board wiring short
Diagnostic Steps
- SAFETY: Close gas supply immediately
- Test gas valve operation
- Inspect board for shorts
Line Voltage Polarity Reversed
Hot and neutral wires are reversed at the furnace power supply.
Probable Causes
- Reversed wiring at outlet or disconnect
Diagnostic Steps
- Check outlet polarity with multimeter or tester
- Correct wiring at disconnect switch
Gas Valve Circuit Failure
Gas valve coil or circuit is not functioning.
Probable Causes
- Bad gas valve coil
- Board gas output failure
- Wiring damage
Diagnostic Steps
- Test gas valve coil resistance
- Check board output voltage to gas valve
- Inspect wiring between board and valve