Furnace no heat - sequence troubleshooting

What is going on

Step through the normal heat sequence: pressure switch check, 30-second prepurge, 8-second ignition trial on low fire, blower on about 20 seconds after ignition, staging, 10-second post-purge.

Likely causes

• No 24V on W
• Inducer or pressure switch problem
• No spark
• No gas / low gas pressure
• Weak flame sense
• Limit or roll-out open
• Blower not starting

Check in this order

1. Set fan to AUTO and call for low heat. If a fault code shows, follow that code.
2. Confirm 24V on W at the IFC; if none, check thermostat, batteries and wiring.
3. Inducer should run 30-second prepurge; if it runs 60 seconds then stops for 5 minutes, the pressure switch did not close - check hoses, vent, inducer.
4. Check spark during the trial; check igniter, wires and placement.
5. If burners light but drop out, clean flame sensor, check polarity and ground, check gas pressure.
6. Blower should start about 20 seconds after ignition; if not, check HEAT-NEUTRAL voltage, wiring, capacitor and motor.
7. For high heat, check 24VAC on W2 and that the high gas valve is energized.
8. Most failures are not the IFC - check wiring and ground before replacing it.

More furnace guides

• Blower codes 66, 61 and 60 - what to look for
• Blower does not start after burners light
• Burners go out before the heat call ends / short cycling
• Communicating system not found / no communication
• ECM blower does not start on heat
• ECM blower will not run - power and communication checks (code 68)