AO Smith Water Heater Won’t Stay Lit [7 Easy Solutions]

There are leading reasons for your AO Smith water heater won’t stay lit including a defective thermocouple, a dirty filter, lack of combustible air, failure of ignition, a defective pressure switch, malfunctioning gas valve, and so on.

Whatever the issues are, first you should concentrate on the cleanliness of the pilot and reset your water heater to resolve them. Then, if you fail to fix the problems, follow our well-proven guide to resolve the pilot light issue effectively by yourself.

AO Smith Water Heater Won’t Stay Lit [7 Easy Solutions]

Here, we will explore the responsive factors behind your AO Smith water heater pilot light not staying lit and guide you on how to solve them instantly. Let’s get started.

Note: You can also read our guide on AO smith hybrid water heater troubleshooting.

1. Dirty Filter/Screen

A dirty filter is primarily responsible for your AO Smith water heater not staying lit. Your heater will starve of oxygen if the air filter or screen gets clogged. As a result, it suffers from a lack of combustible air.

When it starves for air, it gets too hot, and the thermal switch may trip, which doesn’t allow the heater to stay lit at all. Let’s go about solving the problem.


First, open the air vent to see if there is any dirt and debris. After finding any dust, take at least a 12-inch brush and clean the bottom line and brush off the clogged filter.

brush the clogged filter 

Now, place your air tube tightly, open the gas valve, and turn on the water. If you can’t solve the problem yet, then you may have a problem with the thermocouple or flame sensor.

2. A Defective Thermocouple

A faulty thermocouple is one of the major culprits that prevent your water heater pilot from lighting. Basically, the water heater thermocouple is considered the brain of the water heater.

When the Pilot Light is on the AO Smith Water Heater but not the Burner stays lit, it is basically a common issue of a bad thermocouple. Because it can’t detect the pilot flame and prevents your heater from turning on.

AO smith water heater burner

AO Smith Water Heater Burner

On the other hand, once you lighted the pilot but it went out when you released the control knob, the thermocouple may need a replacement. Unfortunately, if the thermocouple fails to detect a flame, the ignition system will struggle to light the main burner.

This may also occur if the part is broken or worn out. It will also clear sign to you that the flame can’t heat the thermopile sufficiently. So, the pilot light won’t stay on unless the thermopile reaches its operating temperature. However, it’s very simple to solve.


To fix it, turn off the gas valve and remove the access cover. Let cool down the unit for 10 minutes. Examine the thermocouple, whether it gets clogged or bad.

pull out the thermocouple

Pull out the thermocouple with a screwdriver.

Ensure the thermocouple has no rust. If it gets rusty, clean it with sandpaper and scrape off the dirt so that the pilot gets enough ventilation. 

But if you notice the thermocouple is bent or bad, it would be the best solution to replace the broken thermocouple. You can read the article on how to replace a thermocouple by yourself.

3. Malfunctioning Gas Controller

Your AO Smith water heater pilot light won’t stay lit even after replacing the thermocouple because of a malfunctioning gas valve.

Because the gas control valve is a vital element of the heater that permits the pilot to stay lit. In fact, the lighted pilot heats the thermocouple that delivers a signal to the valve to operate the burner in order to heat the water.

But, if the gas control valve doesn’t get the signal, it won’t let the pilot light. In most cases, this may happen if the gas valve may be damaged because of a fault in the safety circuit.


To resolve the issue, we recommend you reinstall the water heater control valve to resolve it. Let’s see how to replace a faulty gas control valve.

  • First, turn off the gas supply valve.
  • Disconnect the lines, such as the igniter, pilot tube, thermal switch, or thermocouple from the gas control valve.
  • Use a threaded pipe to detach the gas control valve.
  • Replace a new valve, reconnect control valve port connections, and turn on the gas supply.
  • Make sure there is a perfect balance between the gas and the air supply. Finally, your lighting issue will be solved. 

4. Improper Installation

A faulty installation is a next reason your new AO water heater won’t stay lit. Your heater’s light won’t come on if:

  • You don’t turn the manual gas valve to the ON position.
  • Lack of air in the pilot tube and the gas supply line.
  • Ignition procedures don’t work properly.


  • Make sure you have the right position of the gas supply valve; otherwise, no gas will flow to the burner chamber for combustion.
  • Next, check for sufficient air from the pilot tube and the supply line. To inspect it, press the control knob to let air flow.
  • Finally, investigate to see if the ignition is sparking well. If it doesn’t work, check the ignition process.
  • Have a sharp eye on the electrode or anode rod. You’ll see a spark from there. Don’t touch the pilot assembly. Here, check the anode rod if it gets obstructed by any dirt or rust. Because it can prevent your igniter from igniting. So, if it’s needed, use an abrasive cloth to clean them before replacing the unit.
  • If there is no spark, recheck the wire connections. There is a problem with the igniter. So, replace the defective igniter. 

Note: If you are not good at installing the heater, call a professional to install it properly.

5. Pressure Sensor

The pressure switch is another leading reason behind the heater’s pilot light not igniting. This vital device sends a signal to the control board to run the ignition process. Because it acts as a safety feature on the heater that senses negative pressure created by the draft motor.

Functionally, the pressure switch closes during airflow. But, if the process is delayed, wait until it lights up. But, if it doesn’t light, then the pressure switch has defected, and the water heater won’t light at all.


When the point comes to the broken switch, it is wise to replace the malfunctioning switch by installing a new one. Before replacing, don’t forget to measure the voltage on the element with an ohmmeter. Because a defective switch will read a high voltage, meaning its defection.

6. Failure Igniter

Your AO Smith water heater burner won’t stay lit due to the failure of ignition. To stay lit, it is essential to have enough voltage with the igniter. But when the voltage is fine, but it doesn’t glow, this warns that the ignition is defective.

Even the water heater won’t heat if the pilot light doesn’t light, as it works as a push-button igniter. Besides, over time, carbon deposits can build up on the pilot, thereby creating an obstruction to light the pilot because of lacking sufficient combustion of air.


  • First, ensure the igniter is getting sufficient voltage.
  • Keep in mind, that if the pilot is restricted by any means, try your best to clear it out with a stiff brush or compressed air.
  • And if it gets enough voltage but there is no light at all, the ignitor is defective and replaces the pilot immediately.

Hopefully, your burner will stay lit now.

7. Clogged Flame & Base Ring

Your AO Smith water heater’s pilot light will keep going out frequently due to a clogged flame arrestor or base ring. It is a potential cause if the flame gets dirty because of lint and debris. If it occurs, the flame arrestor will appear discolored.


Clean the base ring filter at least once a year. Follow the steps to clean it:

  • Turn off your heater and let it cool down fully. Use a vacuum cleaner to rub out the debris from the base ring filter. Now, relight the pilot. If you can’t relight, we will show you the effective steps below this article on how to relight.
  • Secondly, check the flame arrestor for a clog. If any dirt or debris gets stuck there, it will slow the flame as the air can’t enter the combustion chamber.
  • To clear it out, remove the burner parts from the unit.
  • Turn off the gas valve and disconnect all the lines from the gas control valve.
  • Remove the burner assembly access panel cover. Now, clean the combustion chamber and arrestor.
  • Lastly, place everything in its position. If you detect any discoloration in the flame, call a professional plumber to inspect what is wrong with the units.

How To Relight Your AO Smith Water Heater

Once you have completed your repairs, you can relight your AO Smith water heater by following these steps:

  • Turn the gas supply off to your heater and leave it for at least 10 minutes to cool down before attempting to relight it.
  • Adjust the thermostat temperature to the lowest setting.
  • Place the gas control knob in the pilot position.
  • Press down on the Red Pilot Set button to send gas to the burner assembly.
  • Hold the Pilot red button gently until you hear a “click” sound, and the pilot light returns. It may take a few attempts to get it started. You may need to press the button a couple of times until the pilot lights come on.
  • Release the button. Set the button slowly after the pilot is lit, then let the thermocouple heat up.
  • Turn on the gas control valve.
  • Replace the back access panel on the back of the water heater.
  • Finally, set your ambient temperature.


If the pilot light isn’t lit at all, your heater won’t be able to keep warm water and instead will experience cold water. Because cold water is a sign that the pilot light has gone out. So, fix it ASAP.

How can I know when my pilot light goes out?

There is a quick or instant sign that indicates your pilot light has failed. Then, you’ll get cold water. You can’t see the flame, or blinking light, rather gets an error code on the screen.

How do I turn my AO water heater pilot light back on?

You will see the knob with the temp setting along with the pilot light. Next, find the ignition button, turn it on, press the ‘Pilot Light’ and push the igniter. You will see the light on.

How can I know whether the gas line has a blockage?

If the burner lights for a short time and goes out again, then your heater’s gas line has been blocked. A partly blocked burner may allow light, but a completely blocked burner prevents it from lighting.

What should I do if the pilot light on my AO water heater goes out?

First, try your best to relight by following the user’s directions on the heater’s label if the AO water heater pilot light goes out.


Hopefully, all the aspects behind the pilot’s light troubles are clear to you. Because we have explored all the leading reasons, why your AO Smith water heater won’t stay lit and shown their instant solutions,

So, follow our guide to resolve your AO water heater lighting issues easily by yourself as guided. Yet, if you have any more queries about the pilot light issues, let us know. Don’t be late to tell us through your comments in the box below. We’ll reach out to you as soon as we can.

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