AO Smith Tankless Water Heater Stuck In Standby Mode [Fixed]

AO Smith Tankless water heater stuck in standby mode mainly due to exceeding the safety level of water temperature. Or, it is an indication that the heater is crossing the usage hot water demand. 

Whatever the reason is, you can simply press the power off button to shut the unit. If that is not proving well, resetting the complete water heater system is a quick diagnosing method to get the water heater back to normal condition.

ao smith tankless water heater stuck in standby mode

Go on reading out this exclusive guide to learn all effective processes to deactivate the standby mode in a shorter time.

What Is Standby Mode On AO Smith Tankless Water Heater

Standby Mode is a setting in your water heater system that allows the unit to stand by until you are switching the power on or off.

what is standby mode on ao smith tankless heater

Mainly when you are turning on the hot water faucet, the cold water enters the system and starts the heating operation. But when the hot water valve is shut off, the heater goes into standby mode.

Despite this normal fact, there are a few major reasons that lead the system stuck in this mode. Have a glimpse at the below point to learn more about this fact:

  • First, the heater operates fluently when the water temperature is going through a balancing level. But when the water temp exceeds the safe level, the safety switch trips and active the standby mode. 
  • Secondly, the heater will get stuck in standby mode, when there is too much hot water demand at a time. If you are using too much hot water or turning multiple taps on at once, it exceeds the usage demand and turns off the system. 
  • Thirdly, in rare cases, some mechanical defects are also responsible for this issue. That includes a bad thermostat or malfunctioning flow sensor. 

Why AO Smith Tankless Water Heater Stuck In Standby Mode [3 Easy Solutions]

Here are the all responsive factors behind your heater’s stuck standby mode, explained with quick solutions. Let’s dive in.

Note: You can also read how to fix navien tankless water heater goes cold.

1. Tripped Thermal Safety Cut-Out Switch

If your tankless water heater is stuck in standby mode and stops heating water, it’s a bet that the thermal cut-off switch of your water heater is tripped off. 

A thermal cutoff switch is the most important safety element, featured on all water heaters to ensure the smooth function of the unit. The core function of that switch is regulating the water temperature. 

ao smith water heater tripped thermal safety cut-out switch

When the water temperature of a water heater rises to unsafe levels or above 180 degrees F, the switch activates and cuts off the power supply to the heater. When the heater is in under these circumstances, it goes into standby mode and stops the gas supply to the unit.

Anyway, the only way to get rid of this problem is to reset the thermal safety switch. Wondering how to do so? Let’s have a look through the below section: 

How To Reset AO Smith Tankless Water Heater

Resetting the AO tankless water heater is quite straightforward. Just follow up on the below step-by-step guide to reset your Tankless Water Heater effectively and get the unit out of standby mode.

reset the ao smith tankless water heater

Step 1: Disconnect Your Tankless Water Heater

Before starting anything, you have to disconnect your water heater from the power connection.

It is the first safety measure as you are going to inspect the unit. Disconnect all the wires that connect the heater to a power source.

At times, the heater is connected to the breaker box. If really so, flip the switch to OFF.

ao smith heater is connected to the breaker box

Step 2: Remove The Cover Plate

After disconnecting the power connection, the next step is removing the cover plate of the heater. Take a Phillips or flathead screwdriver and remove the screws holding the cover plate. 

remove the ao smith water heater cover plate

Then disconnect the cover plate from the connection. After removing it, you will see the interior of the heater.

Step 3: Pull Out The Insulation

Once you have removed the cover plate, you will find insulation in the back. Remove the insulation from the unit. 

 pull out the ao smith water heater insulation

Then inspect it for any type of damaged or moisture build-up as it may lead to other major problems. Contact a plumber to replace the bad insulation.

Step 4: Locate The Reset Button

Next, it’s time to locate the reset button. Most probably, you will find it button on the upper front or right side. 

locate the ao smith water heater reset button

If you cannot find the reset button, go through the manual of your water heater model or manufacturer.

Step 5: Press The Reset Button

When you find the reset button, just press it inward. You will hear a click sound confirming that the resting process is started.

During this procedure, the complete heater unit will restart and reboot. After completing a successful reset, you will find your heater out of standby mode.

Lastly, reassemble the insulation and front access cover. Then restore the power connection to your heater and start operating.  

2. Bad Thermostat

In many cases, you will counter still the same stuck standby mode even after resetting the thermal cut-off switch.

Now the bad thermostat is the main culprit to blame. The thermostat mainly monitors the heater temperature and ensures that the water doesn’t get overheated.

tankless water heater thermostat

But if the thermostat is at fault, it fails to check and regulate the thermostat. That’s the point the control board of the heater misleads and throws the heater into Standby Mode.

How To Fix:

In such cases, you have to inspect and check the performance of the thermostat of your heater to determine whether it is working or not. 

check the performance of the ao smith water heater thermostat

If you suspect that the thermostat is operating well, contact a professional technician to deal with the bad thermostat precisely. 

3. Malfunctioning Flow Sensor

At times, the bad flow sensor inside of the heater is the leading reason for the stuck standby issue on your heater.

Every tankless water heater features a flow sensor that detects the water flow through the system. 

ao smith tankless water heater flow sensor diagram

Whenever your heater is in hot water demand, this sensor identifies the demand and starts the heating process. Here it monitors the presence of water flow and allows electricity to pass through the unit to heat the water.

But in case the sensor is not operating due to clogged up or wearing out, it fails to send the flow signal to the microprocessor controller. The heater stays idle for the lack of command and leads to the standby mode.

How To Fix:

First up, inspect the flow sensor of your water heater. Check to see whether it is clogged with debris. 

ao smith water heater flow sensor

If yes, clear it with a soft cloth or brush. In some cases, you may need to replace the bad sensor to fix the standby mode issue on your water heater.


Why is my AO Smith tankless water heater not getting hot enough?

If your tankless water heater is not heating, be sure the system is overloaded. Reset the unit first. Plus, bad heating elements, worn-out wires, condensation, or low water flow are also some major reasons behind this issue, 

Why does my tankless water heater go cold after a few minutes?

The low water pressure within the unit is the leading reason in case water suddenly gets cold during use. Also,  gas supply interruption, faulty gas valves, failing heating elements, or bad thermostats are responsible for temperature fluctuation in a water heater.

What is a cold water sandwich in a tankless water heater?

The ‘cold Water Sandwich’ is a common phenomenon that most often heater users face. It is mainly water temperature fluctuation of the system. When this happens, you will get warm water initially. Then suddenly the water goes cold and it turns hot after a while.

Are AO Smith tankless water heaters reliable?

Yes! Definitely! Whatever model you are choosing from AO Smith, it will show enough reliability while using and lasts long. From the manufacturer’s recommendation, AO Smith water heaters will last up to 15-20 years, if you are ensuring proper maintenance. 


When your AO Smith tankless water heater stuck in standby mode, it doesn’t function at all which is never being an expected fact to a user.

The heater will never back to its normal state until you deactivate this feature. That’s the point you might get puzzled in finding the corrective solution to fix the issue. 

Our comprehensive guide has nicely covered all the reasons with their best solution. Hope that it was an informative read for you.

Still, don’t delay to ask us any of query through comments regarding this fact! We will be glad to hear you.

About William

William is the founder of He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

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