[Solved] A.O. Smith Tankless Water Heater Stuck in Standby Mode

A. O. Smith offers excellent water heaters that are smart, durable, and efficient for very reasonable prices. However, like any other appliance, your tankless water heater may experience issues, like going into vacation mode or standby mode. If your A. O. Smith water heater gets stuck in standby mode for no reason, how can you fix it?

Your A. O. Smith tankless water heater is stuck in standby mode because of excessive water use. Alternatively, it may be caused by the temperature being above safe levels or thermostat issues.

To exit Standby Mode on your AO Smith tankless water heater, reset the unit by pressing the red button behind the panel cover of unit that is covered by insulation.

In the rest of this article, I will explain why A.O Smith tankless water heaters may be stuck in standby mode and then show you how to fix this issue.

A.O. Smith tankless water heater

1. Excessive Use of Hot Water

Your tankless water heater has a specific capacity, and it struggles to perform beyond it. When you’re using too much hot water simultaneously, like multiple people taking showers in different bathrooms, your tankless water heater may become overloaded.

Excessive use of hot water beyond its capacity may overload your water heater to the point that it might disconnect and go into standby mode. The standby mode means that your water heater won’t be able to heat water at all, so it will essentially become useless.

How To Fix

If your tankless water heater is in standby mode because it is overloaded, you can try to fix it by resetting it. All you need to do is find the reset button on the water heater and press it. This is a relatively easy process that requires only a screwdriver, time, and concentration. Follow these steps to reset your A. O. Smith tankless water heater:

  1. Ensure the water heater is not connected to electricity by flipping the circuit breakers off.
  1. Find the electric panel on the heater and open it, using a screwdriver to unscrew the nuts that hold the cover.
  1. Remove the cover and the insulation that is usually found right behind the cover.
  1. Find the reset button, and press it until you hear a clicking sound, which signals that your water heater has been reset.
  1. Put all the insulation back and screw the cover back on the panel.
  1. Flip the circuit breakers on and then turn on the water heater.
  1. Check if the standby mode is gone.

2. Temperature Is Above the Safety Levels

Your A. O. Smith tankless water heater has a safety mechanism that ensures the temperature doesn’t get too high. This mechanism is called a thermal switch, and its purpose is basically to cut off the gas supply when the temperature reaches a certain level.

If the temperature in your heater has reached the safety level, the thermal switch will work as intended and stop heating. As a result, your water heater may go into standby mode and be unable to heat the water anymore.

A.O. Smith tankless water heater

How To Fix

You can try to make sure that the water inside your tankless water heater will not reach the cut-off point to avoid this issue altogether. However, this isn’t always possible, so if the thermal switch causes the water heater to go into standby mode, you need to reset it.

The thermal switch reset process is the water heater reset process I mentioned above. Get your screwdriver and proceed according to the instructions above. 

3. The Water Heater Thermostat Is Broken

Sometimes your water heater may go into standby mode even if the temperature isn’t close to the cut-off point. When this happens, it means that your water heater’s thermostat may be having problems.

The thermostat reads the temperature using sensors and tells the water heater how to work. If the thermostat is faulty, it may trigger the thermal switch for no reason, putting the water heater in standby mode.

How To Fix

To fix a potentially faulty thermostat on your A. O. Smith water heater, you may try resetting it just in case it’s only a temporary glitch. If resetting doesn’t work, you may be dealing with a thermostat problem. To make sure you can use a multimeter to make sure that there’s indeed a problem.

Here’s a video that shows you how to use the multimeter for this purpose: 

In case of a thermostat issue, there isn’t much you can do except for replacing your water heater’s thermostat with a new one. It’s relatively easy to do by yourself. However, if you’re unsure of being able to perform the replacement, you can also contact a specialist for help.

A.O. Smith tankless water heater thermostat

4. Safety Switch Tripped

The thermal safety cut-out switch is an essential safety feature found on all water heaters.

Its primary purpose is to regulate the temperature of the water, and when it rises to unsafe levels or above 180 degrees F, the switch will activate and shut off power supply to the heater. This causes the heater to go into standby mode and stops the gas supply from reaching it.

If your tankless water heater is stuck in standby mode and not heating up, then it’s likely that this switch has been tripped off.

How To Fix

The Thermal Safety Switch can be reset in the following way.

  1. Go to your circuit breaker which is either in your garage or on the outside walls of your home.
  2. Open it and locate the breaker that is labeled “Water Heater”
  3. Turn the breaker into the off position
  4. Wait for 10 minutes 
  5. Turn the breaker back into the on position

Your thermal switch will now be reset. If this doesn’t work, then you may need to replace it. 

5. Bad Flow Sensor

The flow sensor in your tankless water heater detects the flow of water across the heating elements in the  unit.

When there is no flow, it activates the standby mode of the unit. In other models, it may activate the safety switch which completely shuts off the water heater. 

Flow sensors can go bad over time. 

In order to fix the flow sensor, it has to be replaced. Your water heater manual has the appropriate part number for the particular flow sensor for your unit. 

Other Troubleshooting Steps to Resolve Standby Mode

Sometimes after resolving one issue on your unit, others pop up. AO Smith units are great at displaying the proper error codes for the particular issue you may be dealing with.

Depending on the error you are dealing with, there will be a different path to resolving it. These are the most common places to start when trying to resolve an issue on your system.

  • Start by resetting your system by pressing the red button behind the panel cover
  • Make sure that the breaker to your system is turned to the on position
  • Make sure that the flame is on by looking through the sight glass on the front of your unit


Sometimes, your A. O. Smith water heater may go into standby mode seemingly for no reason and stop functioning properly. There are several reasons why this happens, like excess water usage, a water temperature that is too high, or a broken thermostat.

For most of these issues, the best solution is to reset the water heater by pressing the red button that is located on the electric panel. However, if this method doesn’t work, you may need to replace certain parts of the heater or even the whole thing.