Furnace Troubleshooting
Use this guide to troubleshoot problems, look up error codes, and estimate furnace repair costs.
Start here: No heat diagnostic
Refer to the furnace glossary if you need help identifying furnace parts or terms.
If you have no heat and your furnace simply isn’t running, verify the following about the power supply, filter, and thermostat first:
Furnace filter is clean.
If it's dirty, replace it with a new filter or remove it (for now).
If all of the above check out, continue to find error codes for Mid- and high-efficiency furnaces or troubleshoot a Low-efficiency furnace.
Pilot light is off. It won't stay lit or I can't relight it.
Faulty thermocouple or gas valve.
Burners light, but only briefly. Low heat output. Burnt plastic smell. Grinding or buzzing noise. (Any combination)
Faulty blower motor or capacitor.
Pilot light is on and thermostat calls for heat, but burners do not light. Cold air comes through the vents.
Faulty thermostat or fan limit control.
None of these describe the issue.
Secondary diagnostic
Only do this if directed, after finding the error code for your furnace make and model.
To further diagnose the issue with your furnace, follow these steps:
- Ensure thermostat is on and calling for heat.
- Ensure upper furnace cover is off.
- Turn power switch to the furnace off for 20 seconds, then turn it back on.
- Observe for 5 minutes and select from the descriptions below.
Blower motor runs continuously. Inducer motor does not try to start. Error code may indicate limit/roll out switch is open.
Blocked furnace filter, faulty limit switch, or plugged heat exchanger.
Inducer motor diagnostic
Only do this if directed from another troubleshooting step.
To further diagnose the inducer motor, follow these steps:
- Ensure thermostat is on and calling for heat.
- Ensure upper furnace cover is off.
- Turn power switch to the furnace off for 10 seconds, then turn it back on.
- Observe for 5 minutes and select from the descriptions below. (It’s normal for the blower motor to run for 1-2 minutes before the ignition sequence begins.)
Inducer motor runs for a few minutes, shuts off, and restarts. It sounds like there is water inside the motor.
Inducer motor runs for a few minutes, shuts off, and restarts. Igniter does not glow orange. Error code might be "pressure switch failed to close".
Inducer motor runs for a few minutes, shuts off, and restarts. Igniter does not glow orange. There is a slight natural gas smell.
Inducer motor makes a grinding, buzzing, or humming noise. It does not sound like normal operation.
Failed inducer motor.
Inducer motor runs normally. Igniter glows for 10-30 seconds, but burners do not light. After a few tries, the furnace locks out.
Failed gas valve.
Thermostat diagnostic
If there is a problem with your thermostat or its settings, it may not be calling for heat from the furnace as expected.
To verify the thermostat is calling for heat, see these examples of common thermostats calling for heat, and follow these steps:
- Check or replace the thermostat batteries.
- Set thermostat mode to ‘heat’ or ‘auto’.
- Set fan mode to ‘auto’ (if available).
- Set the desired temperature to at least 2 degrees above the current indoor temperature, as shown on the thermostat.
- Wait up to 5 minutes for the thermostat to ‘call for heat’ and the furnace to turn on. Some thermostats have a built-in delay to prevent short cycling the furnace.
Thermostat calls for heat and furnace runs normally.
Nice work!
Thermostat does not call for heat or is malfunctioning.
Faulty thermostat.
Power diagnostic
Power to the furnace can be interrupted or accidentally switched off in multiple ways. Check the following to make sure:
Electrical panel — Find the circuit breaker for the furnace and ensure it’s switched on.
Power switch — Most furnaces also have a power switch on them or nearby. If you have one, ensure it’s switched on.
Lower furnace cover — For safety reasons, some furnaces automatically disable the power when the lower cover is open. Ensure it is closed and secure.
Some low-efficiency furnaces have a manual switch to run the blower motor (fan). If this doesn’t work, the furnace probably isn’t getting power.
Once you’ve verified that power to the furnace is on, don’t turn it off again while troubleshooting — unless instructed otherwise. Turning off the power clears the error codes used to diagnose furnace problems.
Furnace powers up and is working normally.
Nice work!
Furnace still does not power up.
Filter blockage diagnostic
Only do this if directed from another troubleshooting step.
A dirty furnace filter could be the problem. To diagnose further, follow these steps:
- Ensure thermostat is on and calling for heat.
- Remove the furnace filter.
- With filter out, wait 5 minutes, then reset the furnace by turning the power switch to the furnace off for 20 seconds, then turn it back on.
- Observe for 5 minutes and select from the descriptions below. (It’s normal for the blower motor to run for 1-2 minutes before the ignition sequence begins.)
Furnace powers up and is working normally.
Nice work! Don't forget to replace your furnace filter with a new one.
Inducer motor turns on, igniter glows orange, and the burners light for 1-5 seconds — then abruptly cut out.
Faulty flame sensor/rod, dirty burner assembly, or failed control board.
Furnace runs for 1-20 minutes but shuts off before house reaches set temperature. Error code may indicate an open limit switch.
Blocked furnace filter, heat runs or returns, plugged heat exchanger, or faulty blower motor. Professional diagnosis required.
Burners light, but only briefly. Low heat output. Burnt plastic smell. Grinding or buzzing noise. (Any combination)
Failed blower motor.
None of these describe the issue.
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Furnace repairs should only be performed by a certified technician.
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