Furnace not igniting: no-heat troubleshooting guide for HVAC technicians
Furnace not igniting or producing heat? Troubleshoot thermostat calls, ignition sequence, pressure switch, igniter, flame signal, gas and safeties.
A furnace that will not ignite can stop at many points: no thermostat call, inactive inducer, unproven pressure switch, unpowered igniter, closed gas supply, missing flame signal or an open safety. The fastest diagnosis follows the actual sequence instead of replacing parts by guesswork.
Identify the make and model, obtain the official manual and follow all gas, combustion and carbon-monoxide procedures before testing. A fault code points to a manual section; it never replaces measurements and inspection.
1. Confirm the heat call
Verify HEAT mode, setpoint, schedule, batteries and control voltage. Confirm the W call at the furnace. If the thermostat is calling and nothing starts, check power, service switch, blower-door switch, low-voltage fuse and safeties first.
2. Follow the ignition sequence
- The thermostat sends a heat call.
- The inducer starts.
- The pressure switch proves draft.
- The igniter heats or sparks.
- The gas valve opens.
- Flame is proven.
- The main blower starts after its delay.
Record the exact step where operation stops. That observation immediately narrows the fault.
3. Inducer and venting
If the inducer does not start, verify call, supply, capacitor where applicable and mechanical condition. If it runs without proving the switch, inspect intake and exhaust, condensate drain, trap, tubing, fittings and differential pressure against manufacturer data. Never bypass a pressure switch to operate equipment.
4. No heat or spark at the igniter
Confirm that the board reaches ignition, then measure the specified voltage. With power off, inspect the igniter for cracks and test it as directed. Hot-surface igniters are fragile and can be damaged by handling or contamination.
5. Igniter operates but no flame
Verify the shutoff is open, gas supply and pressure are correct, and the valve receives its command. Gas-train work requires the applicable qualifications and instruments. Do not condemn an energized valve before completing the manufacturer procedure.
6. Flame appears and drops out
A flame lasting only a few seconds commonly indicates a flame-proving issue, but polarity, grounding, connections and combustion quality also matter. Clean and position the sensor only as specified, then measure flame current in microamps.
7. Overheating and open limit
If burners run and stop before setpoint, inspect filter, return, registers, blower, motor speed, evaporator and temperature rise. Compare rise with the nameplate. An open limit is a protection symptom; bypassing it creates a serious hazard.
8. Verify combustion after service
After correction, verify manifold pressure, combustion, draft, venting, temperatures, cycles and carbon monoxide with the required instruments. Confirm building detectors are present and operational under local requirements.
Field report checklist
- Model, fuel and observed fault code.
- Thermostat call and control voltages.
- Exact sequence step where operation stopped.
- Differential pressure and vent/drain condition.
- Flame current, temperature rise and motor current.
- Combustion readings and final operation.
FrigoTechPro connects nameplate scanning, the electrical solver, reports and the HVAC-R AI assistant in one field record. Continue with the blank thermostat guide and the 24V fuse troubleshooting guide.