14 Common AO Smith Water Heater Troubleshooting Codes

This is an in-depth guide on AO Smith Water Heater Troubleshooting Codes. Throughout this article, I will disclose the meaning of the error codes your AO Smith Electric Water Heater throws. 

Besides, I will also walk you through the troubleshooting steps to clear those error messages. So, keep reading. 

Table of Contents

• 14 Common AO Smith Water Heater Troubleshooting Codes
  • Ao Smith Water Heater Error Code E02
  • Ao Smith Water Heater Error Code E03
  • AO Smith Water Heater Error Code E04
  • AO Smith Electric Water Heater Error Code E06
  • AO Smith Electric Water Heater Error Code E10-E13
  • AO Smith Electric Water Heater Error Code E14
  • AO Smith Electric Water Heater Error Code E20-E24
  • AO Smith Electric Water Heater Error Code E51 & E52
• AO Smith Tankless Water Heater Troubleshooting (Error) Codes 
  • Error Code 031
  • Error Code 111
  • Error Code 121
  • Error Code 510, 551
  • Error Code 611, 621 
  • Error Code 741
  • Error Code 991
• FAQs
• End-Note

14 Common AO Smith Water Heater Troubleshooting Codes

This chapter will describe the meaning of a specific error message your AO Smith Electric Water Heater throws. You can see those error codes only on an AO Smith Water Heater with two or four chambers. 

Ao Smith Water Heater Error Code E02

Error code ‘E02’ on AO Smith Electric Water Heater indicates that the unit determines the heated water exceeds the temperature limit.

If you don’t have the Energy Smart® Module installed, take a look at the Electronic Thermostat (ET) for the diagnostic LED. The diagnostic LED will flash red two times.

Like other electric water heaters, AO Smith also features a high-limit switch or electric-cut-off button. 

When it senses the water temperature crosses the safety level, the reset or ECO button cuts the electric supply off. As a result, the heating element stops warming the water. 

Many reasons cause your water heater to heat the water excessively. For example, a bad thermostat, a bad computer board, and burn-out connectors that lead to the computer module are responsible for producing too much hot water.

How To Fix: 

Resetting your water heater is a simple fix to this problem and helps you clear the error message. 

First off, turn the electric supply off to your water heater at the circuit breaker. Then, use a flathead screwdriver to remove the nuts that hold the upper access panel in its place. Remove the access panel and fold away the insulation. Now, press the red button to reset the unit successfully. 

Afterward, put everything back into its place. Turn on the electric supply by flipping up the circuit breaker. Finally, turn on your water heater and you can see the error code is gone. 

Though resetting the unit will fix and clear the error code, you must figure out what’s overheating the water. Typically, a bad thermostat, burn-out heating elements, and bad computer board are some defects that cause this issue. I highly recommend you use a multimeter to test the continuity of them and replace them if needed. It will be far better if you call a certified technician to deal with this issue. 

Ao Smith Water Heater Error Code E03

Your AO Smith Water Heater will flash the error code ‘E03’ or flash red 3 times on the ET if there is a low water level in the heater. If there is air in the waterline, your water heater may throw this erratic code. Sometimes, a faulty sensor is also responsible for this issue. 

How To Fix: 

To solve & clear the error message, turn the power off to your water heater first. Second, turn on a hot water faucet anywhere in your house to let the air out of the waterline. Once you find the water is flowing uninterruptedly, turn off the hot water faucet and turn the power back to the unit. 

If the error code returns, I bet the sensor is at fault. So, replace the sensor to clear the faulty code. If you don’t know how to replace the sensor, call a professional for the replacement. 

AO Smith Water Heater Error Code E04

Your AO Smith Water Heater will throw the error code ‘E04’ flash the LED red four times when it detects moisture or leak in the heater. A water heater leaks from different places like from the top, bottom, and side. When the water leaks out of any place from the water heater, the leak sensor senses it and notifies you by throwing the fault code ‘E04’.  

How To Fix: 

As long as you see this error code, you must turn off the water heater first. Then, find the source of the water leak. Generally, the heating element, top of the unit, tank, and drain valve are some common places from where the water is leaking. You can read this article on Rheem Water Heater Leaking. Though the brand is different, it will help you a lot in finding the leaking point. However, I highly recommend you call a certified plumber to solve this issue. 

AO Smith Electric Water Heater Error Code E06

A faulty code ‘E06’ on your AO Smith Water Heater stands for too hot water temperature. Or, you will find the diagnostic LED on the ET flashing red 6 times if the water temperature is too hot.

In other words, when the sensor senses the water temperature is too high, it will throw this error code.

Generally, a non-functioning thermostat, shorted heating element, and an unfit thermostat against the tank are the culprits that cause the water temperature of your heater too high. 

How To Fix: 

This error code may appear when you power up the unit. Just turn on a hot water faucet and let the water run for several minutes. It will clear the error code. Turns out, ensure you don’t set the thermostat too high. If needed, adjust the thermostat setting. 

Next, check the heating element for damage. If the resistance of this part goes out of range, you must replace it. 

Finally, install the thermostat professionally if it doesn’t fit against the tank snugly. 

AO Smith Electric Water Heater Error Code E10-E13

Your AO Smith Electric Water Heater will throw the error codes E10, E11, E12, and E13 either for an open inlet temperature sensor or open thermistor. Basically, both 2-chamber and 4-chamber models will pop up with those error codes. 

How To Fix: 

Follow the below steps to erase those error codes from the display: 

• Turn the electric power to your water heater off at the circuit breaker.
• Visually inspect the thermistor connections.
• Detach the thermistor and reattach it again. 
• Now, back the electric power to your water heater and it will clear those faulty codes. 

If the error codes return, you must replace the thermistor. The following steps will help you change the thermistor: 

Start with turning off the electric power to your water heater. Then, ensure you have the following pieces of equipment at your fingertips: 

• Philips head screwdriver
• ½-inch open-end wrench
• Plastic bucket
• Siphon transfer pump and towels

Firstly, turn off your water heater. 

Secondly, check the user manual and drain the water heater by following the instructions. 

Thirdly, locate the thermistor on your water heater. This is exactly where the thermistor is located, depending on models: 

Once you locate the thermistor, detach the thermistor wires from the control board. Next, use the ½-inch open-end wrench and pull out the thermistor by turning it counter-clockwise. 

Afterward, install a new thermistor. Verify the O-ring is in its place. Then, hand-tighten the thermistor first. And then, use the wrench to secure the thermistor properly. 

What you need to do is- attach the thermistor wires to the control board. Replace everything back into its place. 

Now, turn the electric power on to your water heater. Open a nearby hot water faucet to let the air out. Keep opening the hot faucets until all the air gets expelled out of the water heater. 

Next, check for water leaks and fix them if needed. Before inspecting for leaks, turn the unit off. Once you solve the leak issue, return the electric power to your water heater. 

AO Smith Electric Water Heater Error Code E14

The 4-chamber model of AO Smith Electric Tankless Water Heater only throws this error code. Generally, the water heater will display this faulty code if the outlet thermistor (TH-4) remains open. 

How To Fix: 

Very simple. Just shut off the electric supply to the unit. Check the thermistor connections. Then, disconnect the thermistor and reconnect it. Finally, turn the power back to the unit and it will clear the error code. If the error code re-appears, replace the thermistor. I already discussed how to make the replacement. So, check the previous chapter again. 

AO Smith Electric Water Heater Error Code E20-E24

If you have either a 2-chamber or 4-chamber model of AO Smith Water Heater, you can see the trouble codes E20, E21, E22, E23, and E24 for a single reason. And the reason is the thermistor gets shorted electrically. 

How To Fix: 

The only solution to clear those error messages is to replace the thermistor. Fortunately, I broke down how you can replace the thermistor correctly in this section: Ao Smith Electric Water Heater Error Code E10-E13. So, check out the section again. 

AO Smith Electric Water Heater Error Code E51 & E52

If your AO Smith Electric Tankless Water Heater throws the error code E51 or E52, understand the user interface lost its connection with the control board. 

How To Fix: 

To fix this issue, you should replace the control board. Before doing so, don’t forget to check the wiring attached to the control board. I don’t recommend you perform this workflow by yourself. Just hire a certified technician to do this for you. 

AO Smith Tankless Water Heater Troubleshooting (Error) Codes

This chapter will explain the meaning of error codes your AO Smith Tankless Water Heater throws, their reasons, and how you resolve them. 

Error Code 031

Your AO Smith Tankless Water Heater will throw the error code 031 on the controller display if you set the DIP switch incorrectly.

The manufacturer company should already set the DIP switches in the right order. 

The problem will arise when the installer installs the unit and fine-tunes the DIP switches. 

How To Fix: 

Fortunately, setting the DIP switches is easy, though most homeowners get panicked. 

On the control board, you will see DIP switches on two sides- the left bank of DIP switches and the right bank of DIP switches. 

I highly suggest you turn off the power supply (both gas and electric) to your tankless water heater before adjusting the DIP switches. 

Then, adjust the DIP switch settings according to the instructions mentioned in the following picture: 

Error Code 111

Faulty code 111 on the AO Smith Tankless Water Heater means ignition failure. The tankless water heater will display the error code 111 because of one of the following reasons, including: 

• Insufficient gas supply or the inlet gas pressure is improper
• Defective Hi-Limit switch
• Loose wire connections or the flame rod is faulty
• Wires connected to the gas valve assembly or the igniter come loose
• The heat exchanger is leaking
• Dirty flame rod 

How To Fix: 

To erase the error code 111, try the following troubleshooting steps:

• Verify that you turn on the gas supply and the gas valve is in the On position. Next, measure the inlet gas pressure and ensure it’s within the specification. For a water heater powered by natural gas, the range of inlet gas pressure should be between 4.8 w.c. On the other hand, the proper inlet gas pressure for a proper tankless water heater should be 8.5″ W.C.-14″ W.C. 

• Replace the Hi-limit switch if it’s not functioning properly. 

• Inspect the wire connections of Part #110, 404, 704, 705, 710, and 719 and ensure they are intact. Clean the flame rod (Part #106) if dirt buildup forms on the flame sensing rod.

• Ensure you hear a spark ignition sound from the burner when the tankless unit is ready for combustion.

• Contact a professional plumber if the heat exchanger is leaking.

• Clean the nozzles of the manifold to remove the accumulation of dirt & lint.

• Replace the wiring of the flame-sensing rod.

Error Code 121

Loss of flame is what represents the error code 121 on your AO Smith Tankless Water Heater. 

The error code 121 typically occurs when your tankless water heater tries to ignite the burner but fails due to flame loss. 

Several reasons, including improper gas supply, tripped hi-limit switch, and leaking heat exchanger, are responsible for the loss of flame. 

How To Fix: 

First, verify that your tankless water heater is getting sufficient gas supply. Regarding this, ensure the gas supply valve is in the On position. 

Second, reset the hi-limit switch if it gets tripped. 

Third, call a certified plumber to resolve this issue if the heat exchanger is leaking. 

Finally, check the flame sensor rod for electric current. Regarding this, use a multimeter to verify that the electric current is flowing through the orange flame rod wire. 

If not, replace the flame sensor rod. 

Error Code 510, 551

Abnormal main and gas solenoid valves are what the error codes 510 and 551 represent on your AO Smith Tankless Water Heater. 

You can generally experience this error code on your tankless water heater due to: 

• Breakage of wires or burn marks on the computer board
• Power supply issue
• Voltage issue on each valve on the gas valve assembly

How To Fix: 

Go along with these troubleshooting steps to erase the error code 510, 551: 

1. Check the wires of part #701 or the sign of burn marks on the computer board. Replace the wiring or contact a professional plumber to replace the control board.

2. Reset the power supply to your tankless water heater.

3. Check the voltage on each valve on the gas valve assembly.

• Between blue wire and light blue wire (#3). (Normal: DC 78 to 100 V)
• Between blue wire and green wire. (#9) (Normal): DC 78 to 100 V) ・ 
• Between blue wire and orange wire (#53). (Normal: DC 78 to 100 V)
• Between blue wire and red wire (#73). (Normal: DC 78 to 100 V)

If all the check points are normal, replace the gas valve assembly. Otherwise, you should replace the PCB.

Error Code 611, 621

The flashing code 611 and 621 on the controller display indicates either the fan motor or the exhaust fan motor is at fault.

Your AO Smith Tankless Water Heater will throw these error codes because of: 

• Loose fan motor connection or if there is dirt buildup in the fan motor or the exhaust fan motor
• Fan motor connectors get frozen or corroded
• Voltage issue on the fan motor

How To Fix: 

First, check the wire connection of the fan motor and ensure that the wiring is intact and secure. Then, release the blockage from the fan motor or the exhaust fan motor.

Second, inspect the fan motor wire connectors for corrosion. Knock the wire connector with a pencil to remove the buildup from the wire connectors. 

And thirdly, check the voltage between the blue wire and each wire of the fan motor during operation. I recommend you prefer check point G1 and G2 in the diagram below and the followings: 

• Check voltage between red wire and blue wire (Normal: DC 110 to 160 V) 
• Check voltage between yellow wire and blue wire (Normal: DC 13 to 17 V)  
• Check voltage between orange wire and blue wire (Normal: DC 2.0 to 6.5 V)

Error Code 741

Miscommunication between the water heater and the remote controller is what the error code 741 indicates. 

AO Smith Tankless Water Heater will display the error code 741 due to the following reasons: 

• Installing the incorrect model type of the remote controller
• The connection between the water heater and the remote controller is incorrect
• Power supply issue
• Improper voltage on the remote controller terminal on the PCB

How To Fix: 

Try these troubleshooting steps to erase the error code 741 on the AO Smith Tankless Water Heater: 

Ensure that you install the correct remote controller, whose model number is 9007603005.

Inspect the connection between the water heater and the remote controller and ensure the wire connection is intact and secure. 

Make sure that the water heater is getting an electric supply. 

Lastly, refer to checkpoint ‘F’ on the wiring diagram below and ensure the voltage on the remote controller terminal on the PCB is between DC 13 and 25 V. 

If the voltage reading is normal, you should replace the remote controller. On the other hand, you should replace the PCB if the voltage reading is out of specification. 

Error Code 991

Imperfect combustion is what the AO Smith Error Code 991 represents. To resolve the imperfect combustion, go along with these troubleshooting steps: 

1. Ensure you feed your tankless water heater with the correct type of fuel. See the rating plate on the side of the unit and fuel up the water heater with the proper gas type (natural gas or propane) based on the rating plate instructions. 
2. Check for restrictions or buildup around the terminator or in the air intake and release the blockage from there. 
3. Ensure adequate clearance is maintained between the intake air terminal and the exhaust terminal. 
4. Verify that the total vent length doesn’t exceed 50 ft and the # of elbows is less than 5Ea. 
5. Make sure that the area where you install your AO Smith Tankless Water Heater is free of flammable objects. 
6. Check the heat exchanger for lint or dirt buildup and clean them if required. 

FAQs

How do I reset my A.O. Smith water heater?

It’s very simple to reset your AO Smith Electric Water Heater by following the below instructions: 

• Ensure you shut off the electric supply to your water heater.
• Remove the upper access panel by using a flathead screwdriver. 
• Fold away the insulation to access the reset button (it’s red). 
• Press the red reset button inward until you hear a click. 
• Put everything back into its place and turn the electricity on. And this will reset the water heater successfully. 

What does ECC code mean A.O. Smith water heater?

ECC stands for Electro-chemical compression. Like other heat pump water heaters, AO Smith Heat Pump Water Heater uses the ECC or transformative solid state technology to reduce the energy usage.

Why is AO Smith's water heater beeping?

Your AO Smith Water Heater will make a beep once it detects a problem to notify you that something is going wrong with the unit.

How to tell if the AO smith water heater is working?

AO Smith Water Heater will throw an error code or make a beeping noise if the unit is not working properly. If it doesn’t throw a trouble code, make no beeping, or flashing light, and understand the water is functioning properly.

End-Note

Trouble codes on the AO Smith Water Heater mean your water heater is not working properly. Fortunately, I broke down every common error code your AO Smith Electric Water Heater shows. As a result, you can figure out the issue quickly and take the necessary steps to fix the problem. Of course, you are always open to reaching out to a professional if you fail to clear the error messages. 

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AO Smith Electric Water Heater Troubleshooting