Limit Switch Open
Probable Causes
- Dirty filter
- Blocked return
- Bad blower
- Limit switch failure
Diagnostic Steps
- Replace filter
- Check ductwork
- Test blower
- Check limit
No Flame Sensed
Probable Causes
- Dirty flame sensor
- Bad igniter
- Gas supply issue
- Control board
Diagnostic Steps
- Clean flame sensor
- Check igniter
- Verify gas
- Test board
High Pressure Switch Open
Probable Causes
- Dirty condenser
- Overcharge
- Restriction
- Fan failure
Diagnostic Steps
- Clean condenser
- Check fan
- Check charge
Low Pressure Switch Open
Probable Causes
- Low charge
- Frozen coil
- Restriction
- TXV failure
Diagnostic Steps
- Check charge
- Check filter
- Check TXV
Compressor Lockout
Probable Causes
- Thermal overload
- Bad capacitor
- Locked rotor
Diagnostic Steps
- Let cool down
- Test capacitor
- Check amps
Control Self-Diagnostics Failure
Probable Causes
- Control board failure
- Power surge damage
Diagnostic Steps
- Power cycle furnace
- Check for proper voltage
- Replace control board if persists
Roll-Out Switch Open
Probable Causes
- Heat exchanger crack
- Blocked heat exchanger
- Gas overpressure
Diagnostic Steps
- STOP - potential CO hazard
- Inspect heat exchanger for cracks
- Check gas pressure
- Manual reset rollout switch
External Low Voltage Short
Probable Causes
- Thermostat wire short
- Bad thermostat
- Wire damage
Diagnostic Steps
- Disconnect thermostat wires
- Check for shorts at furnace terminals
- Inspect wire routing
Blocked Vent/Pressure Switch
Probable Causes
- Blocked flue/intake
- Failed pressure switch
- Inducer motor failure
- Condensate drain blocked
Diagnostic Steps
- Inspect flue pipe and intake for blockage
- Check condensate drain
- Test pressure switch
- Measure inducer amps
Ignition Failure / No Flame Detected
Probable Causes
- No gas supply
- Dirty flame sensor
- Bad igniter
- Gas valve failure
Diagnostic Steps
- Verify gas is on
- Clean flame sensor
- Check igniter glow
- Test gas valve voltage
Limit Switch Circuit Lockout
Probable Causes
- Repeated high temp trips
- Chronic airflow issue
- Oversized unit
Diagnostic Steps
- Must manually reset
- Check airflow thoroughly
- Inspect ductwork
- Verify unit sizing
High Limit Switch Open
Probable Causes
- Dirty filter
- Blocked return air
- Bad blower motor
- Oversized gas orifice
Diagnostic Steps
- Check and replace filter
- Verify return air is open
- Test blower motor
- Check gas pressure