The gas valve controls the flow of natural gas to your furnace’s burners. Common signs it is failing include an ignitor that glows but burners that never light, burners that light and quickly shut off, repeated ignition lockout codes, or a furnace that clicks but will not ignite. Many of these symptoms are actually caused by other parts, like a dirty flame sensor, a weak ignitor or a pressure switch, so a technician should test the valve before replacing it. Replacing a furnace gas valve is not a DIY job. It requires leak testing and setting gas pressure with specialized tools, including adjustments for Colorado’s altitude.
When a furnace will not light, the gas valve tends to get blamed first. Sometimes it really is the culprit. Often it is not. Knowing the difference saves Castle Rock and Lone Tree homeowners from paying for parts they did not need, and keeps everyone safe around a component that handles natural gas.
What the gas valve does in your furnace
Every time your thermostat calls for heat, a gas furnace runs through the same sequence of operations:
- The thermostat sends a call for heat to the control board.
- The inducer motor starts and pulls air through the combustion chamber.
- The pressure switch confirms the venting is clear.
- The hot surface ignitor heats up and glows.
- The gas valve opens and sends gas to the burners, which ignite.
- The flame sensor confirms there is a flame. If it does not sense one within a few seconds, the board closes the gas valve for safety.
- The blower turns on and pushes warm air through your ducts.
The gas valve only opens when every step before it checks out, and it shuts the instant the flame sensor loses the flame. That is why so many unrelated problems look like a gas valve problem.
Signs of a bad gas valve on a furnace
- The ignitor glows bright orange, but the burners never light
- The furnace clicks or hums at startup without igniting
- Burners light, then shut off within a few seconds and will not stay lit
- The furnace locks out with ignition failure codes (for example, the alternating red and green flash on many Lennox boards; see our Lennox furnace light codes guide)
- Weak or uneven burner flames
- The furnace runs, but rooms never quite reach the thermostat setting
- A gas smell near the furnace. Leave the house and call 911 or your gas utility from outside.
Problems that look like a bad gas valve but usually are not
| What you notice | What is often the real cause |
|---|---|
| Burners light, then shut off in a few seconds | A dirty flame sensor. Cleaning it is a routine tune-up step. |
| Ignitor never glows | A cracked or worn hot surface ignitor, or a control board issue |
| Nothing happens after the inducer starts | A pressure switch problem, blocked vent pipe or clogged condensate drain |
| Ignitor glows but no gas flows | The manual gas shutoff is closed, low gas supply, or the board is not sending power to the valve |
| Furnace works sometimes, then locks out | Low gas pressure, a weak flame signal, or venting issues |
A technician confirms a bad gas valve by testing whether the control board is sending voltage to the valve and measuring gas pressure at the valve. If the valve is receiving the right signal and still is not delivering gas correctly, then it is the valve.
Should the gas valve on my furnace be on or off?
During heating season, it should be on. Most furnaces have two places to check:
- The manual shutoff valve on the gas line leading to the furnace. When the handle runs parallel (in line) with the pipe, it is open. When it is perpendicular (crossways), it is closed.
- The control knob or switch on the furnace’s gas valve inside the burner compartment, marked ON and OFF. It should be set to ON.
If either was turned off over the summer or during other work, follow the lighting instructions printed on the label inside your furnace. If you are not comfortable doing that, or if you smell gas at any point, stop and call a professional.
Why replacing a furnace gas valve is a job for a licensed technician
- Leak testing: every gas connection that is opened has to be leak tested before the furnace runs again.
- Setting gas pressure: a new valve has to be adjusted to the furnace manufacturer’s specification using a manometer. Too high or too low affects safety, efficiency and the life of the heat exchanger.
- Altitude: at Front Range elevations, gas pressure and combustion have to be set correctly for altitude. Castle Rock sits above 6,000 feet, and furnace settings that work at sea level are not right here.
- Matching the right part: gas valves are specific to the furnace model and fuel type. The wrong valve can cause unsafe operation.
- Codes and warranty: gas work is regulated, and DIY gas work can void your furnace warranty and create insurance problems.
Replace the gas valve or replace the furnace?
On a furnace that is under about 15 years old and otherwise in good shape, replacing a failed gas valve is usually a solid repair. On an older furnace that has also needed an ignitor, a blower motor and a control board in recent years, it is worth stepping back and weighing the whole system. Our guide to furnace repair or replacement walks through that decision.
Furnace gas valve questions, answered
If your furnace will not light, PS Cool Heat can diagnose the real cause and fix it safely. We provide furnace repair in Castle Rock, furnace repair in Lone Tree, and service throughout Highlands Ranch, Parker and the Denver Metro area, 24/7 for heating emergencies. Call 720-613-2210 or request service online.
