AO Smith Water Heater Code E02 - Fixed

The AO Smith water heater code E02 happens quite often and needs to be immediately addressed. While the E02 error code on the AO Smith water heater doesn’t pose as serious of a safety risk as E01, you should nonetheless investigate its possible causes and use the appliance only after fixing the issue.

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An AO Smith water heater code E02 is usually displayed when the high limit switch trips. This happens if the water temperature is too high (which may be due to a user setting). There may also be a defective thermostat, shorted heating element, or bad control module.

Generally, when you see an E02 error, the AO Smith water heater would have already stopped running. You’ll probably find a tripped high limit switch you must reset for the water heater to work again. But if, after doing that, the error code reappears and your heater doesn’t work, you may need to read this article further to resolve the issue.

1. The Water Temperature Is Set Too High

The high limit switch is a safety mechanism that trips and shut down your AO Smith water heater when its temperature is too high. This component is also known as the:

High or hi-limit switch
Emergency cut-off switch
Water heater reset button
High limit safety thermostat switch

Your AO Smith water heater may have a default temperature of 120°F (49°C), which is a factory setting. You can increase this temperature, but there’s a risk of overheating that will trip the high limit switch.

Usually, AO Smith water heaters’ high limit switches trip at 150°F (65°C). You need to check if you have increased the thermostat setting or temperature too much or if there’s another issue at play.

How To Fix

If you have an E02 error code on the AO Smith water heater, you’re probably using an Energy Smart Module. Check the temperature setting in Fahrenheit or Celsius to see if it’s too high.

Here’s how you can change the water temperature setting:

Check the main operational modes at the top of your Energy Smart Module.
Look for the Standard mode, which is usually the second of the 4 modes from the left. The Standard mode allows you to increase or decrease the water temperature.
Reduce the water temperature if it’s too high — i.e., 140°F or 150°F (60°C or 65°C).

An AO Smith water heater’s operating temperature range is 80°F or 150°F (approx. 26°C or 65°C). I recommend the factory setting or default of 120°F (49°C).

But if you have to increase the water temperature, try to stay well below 150°F (65°C) so the high limit switch doesn’t trip.

Changing the temperature setting won’t be enough if the high limit switch is tripped. You must reset it before restarting the water heater (more on this below).

2. One or Both Thermostats Are Defective

Suppose your water temperature setting isn’t too high. In that case, the AO Smith water heater code E02 could be due to a bad thermostat.

thermostat on water heater being changed

Upright electric AO Smith water heaters have 2 thermostats corresponding to the heating elements below:

Electronic thermostat (for the upper element)
Thermistor (for the lower heating element)

If either of the two temperature sensors is defective, the high limit switch may trip, and you’ll see an E02 error code on the AO Smith water heater.

How To Fix

Here are all the steps you need to take to fix a defective thermostat or thermistor.

Turn off the double-pole circuit breaker for your electric AO Smith water heater.
Get a tester to ensure there’s no power at the terminals on top of the upright water heater. You can also remove the plug from the wall receptacle, as you’re dealing with a 240V circuit.
Use a screwdriver to remove the top or upper access panel of your water heater.
Handle the insulation gently to access the high limit reset button.
Press the high limit switch to reset it. You’ll hear a ‘click’ if it’s tripped.
At this point, you can restart the water heater and see if the error code reappears. The code won’t show up if the temperature setting and all related parts are alright. But you’ll probably see the E02 error code again if either thermostat is defective. The high limit switch tripping is a symptom — not a cause per se.
Remove the cover behind the insulation to access the electronic thermostat.
Get a multimeter to test the upper thermostat terminals for electrical continuity. You’ll find four terminals to the left and three to the right on the upper electronic thermostat. The two lower terminals among the four on the left are for the electronic thermostat. The second thermostat terminal, which is the lowest, is connected to the heating element.
Remove all the wires from the terminals and test the thermostat for continuity.
Place the probes on the thermostat terminals (third and fourth on the left side of the module). If you have continuity, this electronic thermostat is working fine, so check the other.
Place one multimeter probe on the 3rd terminal to the left side of the module, like before. Place the other probe on the lowest terminal to the right side, which is for the thermistor. These two terminals shouldn’t have continuity if the water inside the heater is cool.
Use the screwdriver to remove the lower access panel to test the thermistor’s continuity. The lower module is simpler because there are only 2 terminals on the thermistor unit. Check these terminals for continuity, and you should have a reading if the water is cold.
If either or both thermostats don’t have continuity, you must replace them accordingly.

Remember to match the part number before you buy a new electronic thermostat or thermistor. The electronic thermostat includes the high limit switch, as the two are integrated into the module. But if both thermostats are working fine, you may have a grounded or shorted heating element.

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3. A Heating Element Is Shorted (Grounded)

A shorted heating element may spike the temperature for the electrical and electronic parts in the appliance. Therefore, the high limit switch may trip, or the thermostats may not function properly. Either heating element may be grounded, causing the AO Smith water heater error code E02.

How To Fix

You must test the heating elements. If you’ve taken the steps above, you’ll find the terminals of the heating elements below their respective thermostats.

Here’s how you can test the heating elements thoroughly:

Turn off the circuit breaker, and remove the plug from the wall outlet.
Remove the upper and lower access panels, as I explained above, for the thermostats. The heating elements are below the electronic thermostat and thermistor, respectively.
Remove the wires from the terminals to test the heating elements for continuity. Alternatively, you can use the ohms setting on your multimeter to test for the elements’ resistance. These heating elements typically have a resistance in the range of 10 to 15 ohms.

If the elements have continuity or resistance, you must check for an electrical short as follows.

Place a multimeter probe on one terminal of either the upper or lower heating element. Place the other probe on the metal body of your electric AO Smith water heater. If you get continuity, the heating element you are testing is shorted or grounded. If there’s no continuity, check the other terminal of the same heating element for a short.
Repeat the same process for both heating elements as either could have a short.
Replace the shorted or grounded heating element.

4. The Module May Have Burnt Connectors

The electronic thermostat may have burnt connectors causing a temperature spike. A defective control board or computer module with damaged or burnt components may be the reason you’re getting the E02 error code on the AO Smith water heater. However, this problem isn’t as common as the others I’ve already listed.

How To Fix

Replace any burnt, damaged, or failing component. Get a new control board or module if that is the problem. Also, check your warranty if the faulty part is covered. You can also contact AO Smith to explore your options because a control board isn’t as affordable as a thermostat.

Summary

Check the selected water temperature when you find an AO Smith water heater error code E02. Reset the high limit switch and test the thermostat, thermistor, and heating elements if the error code reappears. Finally, check the control board or module and replace the faulty component as needed.