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Water heater not lighting

01 - Tripped Limit Switch or Thermostat

Most limit switches are resettable. If the limit switch or thermostat has a reset button on the back you should be able to reset it. If the button is popped out the limit switch or thermostat has tripped. You can try resetting it by pressing the button. You should hear a click when it resets. You can check the limit switch or thermostat for electrical continuity with a multimeter.
Top Solution

02 - Water Heater Gas Valve and Thermostat Assembly

The gas valve opens to allow gas to enter the burner. If the gas valve does not open, the burner will not light. First, ensure that the ignition source is working properly. Second, ensure that the gas valve has pressure. If the ignition source is working, and the gas valve has pressure, consider replacing the gas valve and thermostat.
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Highly Likely

03 - Remove flammable vapors from the area

Flammable vapors can be drawn into the water heater from paint cans, staining cans, or fuel containers stored too close to the water heater. This can cause the flammable vapor sensor to trip. Make sure any paint cans or other containers that can produce flammable vapors are stored far away from the water heater and vent the area of the water heater if necessary.
Highly Likely

04 - Water Heater Gas Valve Assembly

The gas valve opens to allow gas to enter the burner. If the gas valve does not open, the burner will not light. First, ensure that the ignition source is working properly. Second, ensure that the gas valve has pressure. If the ignition source is working, and the gas valve has pressure, consider replacing the gas valve assembly.
Common

05 - Water Heater Temperature Sensor

Flammable vapors should always be kept away from the water heater. If your water heater has a flammable vapor system (FVIR), sometimes identified as a temperature sensor, the sensor may have tripped which will prevent the burner from lighting. It is recommended to replace a tripped sensor rather than reset it. See owner's manual for more information.
Common

06 - Water Heater Igniter

Some water heaters use an igniter to ignite the gas. If the igniter fails, the burner will not light. To determine if the igniter is defective, first make sure the igniter is getting voltage. If the igniter is getting voltage, but it doesn’t glow, this indicates that the igniter is defective and should be replaced.
Common

07 - Water Heater Draft Inducer Motor

The draft inducer fan motor draws air into the burner and then exhausts it out the flue. As the air flows through the draft inducer assembly, it closes a vacuum switch to signal that proper air flow is present. If the inducer motor is not working properly, the water heater's burner won't light. Over time, the bearings on the inducer motor wear out. The inducer motor could also have a clogged vacuum port. If the vacuum port is clogged, the motor will not be able to close the vacuum switch, preventing the ignition process from continuing.
Common

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