AO Smith Water Heater Pilot Light Keeps Going Out – Fixed

If your AO Smith water heater pilot light keeps going out, it’s a good sign that there’s an issue with your water heater. However, before you can fix it, you first need to figure out what the problem is.

Your AO Smith water heater pilot light keeps going out due to a tripped or broken thermal switch, bad thermocouple or thermopile, dirty air filter and flame arrestor, low oxygen, or backdraft in the flue hood. Alternatively, you may have low gas pressure or a faulty gas valve.   

To figure out which of these is the issue, you’ll need a few tools, including a multimeter and manometer, to test some of the water heater’s critical parts. You should also keep the user manual handy – causes can vary depending on whether you have a mechanical or electronic water heater. If you’re still wondering how to check out your AO Smith water heater when the pilot light keeps on, read on for more information.

ao smith pilot light

1. The Air Filter and Flame Arrestor Are Clogged

All AO Smith gas water heaters have an air filter and a flame arrestor. The specific type of air filter and the structure or alignment of the flame arrestor depends on the model. For instance, an AO Smith water heater with a round base doesn’t have the same air filter as a model with feet.

Regardless of the differences among models, a dirty air filter and clogged flame arrestor prevent sufficient airflow to the combustion chamber of an AO Smith gas water heater. This problem can shut the burner prematurely, or the pilot light can go out due to a lack of oxygen.

How To Fix

You must clean the air filter and flame arrestor. A water heater is often in the basement, attic, or laundry room, and these spaces are vulnerable to dust, lint, and debris buildup. Hence, the air filter and flame arrestor of water heaters need routine cleaning as a part of preventive maintenance. 

2. The Thermocouple or Thermopile Isn’t Working

If the air filter is clean and the flame arrestor isn’t clogged, the next suspect should be the thermocouple or thermopile. Both serve the same purpose, and here’s how to distinguish them:

  • A mechanical AO Smith water heater has a thermocouple. 
  • An electronic AO Smith water heater has a thermopile. 

If the thermocouple or thermopile isn’t working, it won’t open the gas valve in the main control unit. A thermocouple or thermopile generates electrical energy when it gets heated by the pilot light. The gas valve won’t open if the electrical current isn’t generated or the voltage is too low. 

You can test this voltage with a multimeter. Here’s what you need to do:

  1. Get someone to help because you’ll need more than a pair of hands.
  2. Remove the thermocouple or thermopile from the main control unit.
  3. Ignite the pilot light and keep holding the knob down to allow gas flow.
  4. Set the multimeter to millivolts and inspect the sensor with the probes.
  5. Place one multimeter probe at the end of the thermocouple or thermopile.
  6. Place the other probe on the thermocouple or thermopile’s copper wire.
  7. A good thermocouple should read 10 to 30 millivolts on the multimeter. Any reading below 10 millivolts means the thermocouple isn’t working.
  8. A working thermopile must have a reading of 350 millivolts or higher. Any reading below 350 millivolts implies the thermopile is broken.   

How To Fix

You will need to replace a bad thermocouple or thermopile. Many AO Smith water heaters have an assembly that includes the pilot light. So, you may have to get this thermocouple or thermopile and pilot light assembly. However, if these parts are fine, you’ll need to check the thermal cutoff switch.

thermocouple ao smith water heater

3. The Thermal Cutoff Switch Is Tripped or Broken

A thermal cutoff switch is one of the safety features in AO Smith water heaters. The switch won’t complete the electrical circuit if it is tripped or broken, even if the pilot light is on and the thermocouple or thermopile is alright.

If the switch keeps the circuit open, the thermocouple or thermopile won’t be able to activate the gas valve. This means that the burner won’t come on, and the pilot light will go off. 

A thermal cutoff switch trips when the temperature inside the combustion chamber exceeds the maximum threshold. So, this fault is essentially a symptom. Your AO Smith water heater may be starved of oxygen or airflow or overheating for other reasons.

How To Fix

You must reset the thermal cutoff switch. Most AO Smith water heaters made in the last decade or so have a button on this switch. You’ll find the switch near the inspection window or sighting glass. Press the button and wait till you hear or feel a click. 

If a reset doesn’t work, you must test the switch for continuity. Here are the steps:

  1. Unplug the AO Smith water heater from the wall outlet or receptacle.
  2. Get your multimeter and set it to least resistance or continuity mode.
  3. Remove the wires from the terminals of the thermal cutoff switch.
  4. Keep the multimeter probes on the terminals and test for continuity.
  5. If you hear a beep or there’s some reading, the switch is working fine. No reading, open loop, or infinite resistance means the switch is bad. 

If the thermal cutoff switch is broken, it will need to be replaced. 

Old models of AO Smith water heaters have a thermal switch inside the combustion chamber. Such switches don’t have a reset button, but they restore the electric circuit as the temperature drops.

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4. The Main or Manifold Gas Supply Pressure Is Low

Low gas pressure could be an issue, whether in the main line or the manifold supply. Every AO Smith water heater has the required gas pressure levels mentioned on the main control or gas valve unit. Refer to this information and test the gas supply pressure using a manometer. 

How To Fix

Measure both the main supply and manifold pressures. The main supply pressure should be greater than the minimum requirement for the manifold tube. Furthermore, you shouldn’t use propane or LPG for natural gas water heaters – the recommended pressures aren’t identical for natural gas and propane. 

5. The Flue Hood Has Negative Air Pressure or Backdraft

The hot exhaust gasses from the combustion chamber should naturally flow upward into the flue hood and out of your house. Backdraft or negative air pressure can prevent the flue gasses from flowing up and out. So, the pilot light keeps going out due to a lack of oxygen or combustion air. 

How To Fix

Conduct a draft test to confirm if there’s negative air pressure in or around your AO Smith water heater. Light an incense stick or maybe a cigarette and hold it close to the flue hood above your water heater, and check if the smoke flows toward the flue hood and then upward. 

Hold the incense stick, cigarette, or burning paper just outside the rim, not directly under the flue hood. If the smoke doesn’t flow to the hood and up, you need to address negative air pressure or backdraft in the room. Here are a few remedies:

  • Don’t use exhaust fans and blowers directing airflow away from the water heater.
  • Ensure the water heater has adequate fresh air flowing into the room or closet.
  • A centralized HVAC system should have the air return duct in the same location.

6. The AO Smith Water Heater’s Gas Valve Has Failed

An AO Smith water heater pilot light is likely to go out due to a failed gas valve.

This problem is uncommon but not improbable. You can check the manifold pressure using the test port in the main control unit or gas valve. However, keep in mind that such a test isn’t conclusive evidence that the gas valve is working flawlessly. The reading only tells you the gas supply pressure. 

ao smith gas valve on water heater

How To Fix

Most AO Smith water heaters have the gas valve inside the main control unit, which also has the thermostat, knob, striker, etc. Thus, you have to replace this entire component. Mechanical and electronic units are different, so you must match the part number and get a new one.

Final Thoughts

While you should investigate all the issues I have shared in this article, make sure to inspect the wires and connections and check if they are loose, frayed, or burnt. Furthermore, air and gas pressures aren’t static all the time. The pilot light may go out if too many gas appliances are on simultaneously or when there is a significant change in the air pressure inside the room (or throughout your house).