Limit Circuit Lockout
Probable Causes
- Dirty filter
- Blocked return
- Bad limit switch
- Blower failure
Diagnostic Steps
- Check filter
- Verify airflow
- Check limit switch
- Test blower motor
Ignition Lockout
Probable Causes
- No gas supply
- Bad igniter
- Flame sensor dirty
- Gas valve issue
Diagnostic Steps
- Verify gas supply
- Check igniter
- Clean flame sensor
- Test gas valve
Gas Heating Lockout
Probable Causes
- Invalid flame signal
- Flame rollout
- Gas valve stuck open
Diagnostic Steps
- Check flame sensor
- Inspect burners
- Test gas valve
Pressure Switch Stuck Open
Probable Causes
- Blocked flue
- Bad inducer
- Cracked heat exchanger
- Bad pressure switch
Diagnostic Steps
- Check flue
- Verify inducer operation
- Inspect pressure switch tubing
Secondary Voltage Fuse Open
Probable Causes
- Short circuit
- Bad transformer
- Thermostat wiring issue
Diagnostic Steps
- Check transformer
- Inspect wiring
- Test for shorts
High Pressure Lockout
Probable Causes
- Dirty condenser
- Restriction
- Overcharge
- Condenser fan failure
Diagnostic Steps
- Clean condenser
- Check fan
- Check refrigerant charge
Low Pressure Lockout
Probable Causes
- Low refrigerant
- Frozen evaporator
- Restriction
- Bad TXV
Diagnostic Steps
- Check charge
- Check filter
- Verify evaporator airflow
Compressor Thermal Lockout
Probable Causes
- High head pressure
- Low airflow
- Locked rotor
Diagnostic Steps
- Check condenser
- Check evaporator airflow
- Test compressor
Ignition Proving Failure
Probable Causes
- Weak flame signal
- Dirty flame sensor
- Low gas pressure
- Cracked igniter
Diagnostic Steps
- Clean flame sensor with fine steel wool
- Check gas pressure at manifold
- Verify proper grounding
- Test flame signal in microamps
Blower Motor Fault
Probable Causes
- Bad blower motor
- Bad run capacitor
- Wiring issue
- Control board fault
Diagnostic Steps
- Test blower motor windings
- Check capacitor with meter
- Inspect wiring connections
- Test at board
Inducer Motor Fault
Probable Causes
- Bad inducer motor
- Blocked vent
- Control board issue
Diagnostic Steps
- Test inducer motor
- Check vent pipes
- Verify power from control board
Blower Running After Power Interruption
Probable Causes
- Normal after power outage
- Control board memory reset
Diagnostic Steps
- Power cycle the furnace
- Wait 3 minutes and restart
- Check for proper voltage
Lockout - Control Circuit Failure
Probable Causes
- Bad control board
- Voltage spike damage
- Short circuit
Diagnostic Steps
- Power cycle furnace
- Check low voltage wiring
- Test transformer output
- Replace board if persists