G1
Universal HVAC System
Severity: CriticalWhat Does This Error Mean?
G1 on an HVAC system typically indicates a gas valve fault or gas ignition failure. The system tried to start the heating cycle but the gas valve did not open correctly, or the igniter could not light the gas. This is a safety-critical error — the system locks out to prevent unburned gas from building up. Do not attempt to override this error. Call a licensed HVAC technician.
Affected Models
- Gas furnaces
- Gas boilers
- Gas heat pumps
- Combined HVAC systems with gas heat
Common Causes
- The gas valve has failed electrically and is not opening when the control board commands it to
- The igniter (hot surface igniter or spark igniter) has burned out and cannot light the gas
- The gas supply to the system has been interrupted or the shutoff valve is closed
- The flame sensor is dirty or failed — it cannot confirm ignition and shuts down the valve as a safety measure
- The control board has failed and is not sending the correct signal to the gas valve
How to Fix It
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Do not attempt to restart the system repeatedly. If G1 appears and the system locks out, restarting it multiple times without fixing the root cause can allow gas to accumulate.
Most systems allow one or two restart attempts after a lockout. Additional attempts without a successful ignition will put the system into a longer lockout for safety.
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Check the gas supply. Verify that the manual gas shutoff valve near the furnace is fully open (handle parallel to the pipe). Check that the main gas supply to the building has not been interrupted.
If other gas appliances (stove, water heater) are also not working, the main gas supply to the building may be interrupted. Contact your gas utility.
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Check the furnace filter. A severely clogged air filter restricts airflow, which can trigger safety shutoffs that appear as ignition errors. Replace the filter if it is visibly dirty.
A blocked filter reduces airflow through the heat exchanger. The system overheats and shuts down before ignition completes.
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Check the venting. Make sure the exhaust flue pipe and combustion air intake are not blocked by snow, ice, debris, or bird nests.
A blocked flue prevents combustion gases from escaping and triggers a pressure switch safety shutoff that looks like an ignition error.
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Call a licensed HVAC technician. The technician can clean the flame sensor, test the gas valve, and check the igniter and control board safely using proper equipment.
Flame sensor cleaning is a common annual maintenance task. A dirty flame sensor is the most frequent cause of G1 ignition errors.
When to Call a Professional
Gas heating errors are always a job for a licensed HVAC technician. Do not attempt to bypass the gas valve or override the lockout. An unlit gas valve can allow gas to accumulate, creating an explosion and asphyxiation risk. Call a professional immediately if you smell gas — do not turn on any electrical switches and leave the building.
Frequently Asked Questions
I smell gas near my furnace. What should I do?
Leave the house immediately. Do not turn any electrical switches on or off. Do not use your phone until you are outside. Once outside, call your gas utility's emergency line and 911. Do not re-enter the building until cleared by emergency services. A gas smell near the furnace is a serious emergency — treat it as one.
How long does a furnace igniter last?
Hot surface igniters typically last 3 to 5 years. Spark igniters can last longer but require proper gap adjustment. An igniter that is starting to fail will cause intermittent G1 errors — the furnace starts on some attempts but not others. Replacing the igniter is a quick, inexpensive repair ($30 to $60 for the part).
Can I clean the flame sensor myself?
A homeowner can clean the flame sensor if they are comfortable working safely around gas appliances with the system fully shut off and gas supply closed. The flame sensor is a small metal rod in the burner assembly. Remove it, clean it with fine steel wool or emery cloth, reinstall. However, if you have any uncertainty about working on gas equipment, call a technician.