How to Fix AO Smith Water Heater Error Code E13: Heat pump thermistor failure (Full Guide)

The AO Smith Error E13 indicates a heat pump thermistor failure, meaning the sensor responsible for monitoring temperatures within the heat pump assembly is providing an inaccurate reading or has failed entirely. This prevents the energy-efficient heat pump mode from operating, often forcing the unit into backup electric heating or disabling it completely.

Hey there, DIY friend! I know seeing an error code on your water heater can feel like a major headache, especially when you’re looking forward to a warm shower. You might notice your water is only lukewarm, or perhaps the unit is making a clicking sound but the fan won’t kick on. Don’t panic! While “thermistor failure” sounds like a mouthful, it’s usually just a simple sensor issue. We can walk through this together and get your hot water back and running. You’ve got this!

Symptoms of Error E13

Before we dive into the guts of the machine, let’s confirm what you are seeing. The most obvious sign is the E13 code flashing on your digital display panel, often accompanied by a persistent beeping. You will likely notice that the unit has stopped using its heat pump mode (which is the quiet hum and the cool air blowing from the top). As a result, your water may be significantly colder than usual, or the unit might be “short-cycling,” where it tries to start up but shuts down almost immediately. In some cases, the heater will automatically switch to “Electric Mode,” which keeps the water hot but causes a noticeable spike in your monthly energy bill because it isn’t running efficiently.

Comprehensive Repair Guide

Alright, let’s get our hands dirty! Follow these steps carefully, and remember to take photos of the wiring before you unplug anything—it’s a lifesaver when you’re putting it back together.

1. Safety First – Power Down: Before you touch a single screw, go to your home’s breaker panel and switch off the circuit breaker dedicated to the water heater. Warning: Water heaters use 240V electricity, which is extremely dangerous. Verify the power is off by checking the display—if it’s blank, you’re safe to proceed.
2. Remove the Upper Shroud: Use your Phillips head screwdriver to remove the screws holding the plastic cover (shroud) on top of the water heater. Carefully lift the cover off. You may need to disconnect the wire harness leading to the display screen to fully set the cover aside. Be gentle; these plastic clips can be brittle!
3. Locate the Heat Pump Thermistors: You will see a cluster of wires leading to the evaporator coils (the radiator-looking part). Look for two thin wires (usually black or yellow) clipped directly onto the copper tubing. This is your target. Trace these wires back to the main control board.
4. Test with a Multimeter: Unplug the thermistor connector from the control board. Set your multimeter to the “Ohms” ($\Omega$) setting. Place your probes on the metal terminals inside the plug. At room temperature (around 77°F), you should see a reading of approximately 10k Ohms. If the meter reads “OL” (Open Loop) or “0,” the sensor is definitely dead and needs replacement.
5. Replace the Sensor: If the test failed, unclip the old thermistor from the copper piping. Snap the new thermistor into the exact same spot. Ensure it has a tight connection against the pipe; if it’s loose, it won’t read the temperature correctly. Plug the new wire harness back into the control board until it “clicks.”
6. Reassemble and Test: Reattach the display wire and screw the shroud back on. Turn the breaker back on. The unit will perform a self-test. If the E13 code is gone, you’ve successfully fixed it! Give the heat pump about 10 minutes to kick in and start blowing cool air.

• Difficulty: Intermediate (Requires basic electrical testing)
• Estimated Time: 45 to 90 minutes
• Tools Needed: Phillips head screwdriver, Digital Multimeter (for testing resistance), Needle-nose pliers, and a flashlight.
• Estimated Cost: $25 – $65 (depending on if the sensor needs a full replacement).

Technical Explanation of the Fault

To fix the problem, we first need to understand why it happened. The thermistor is a “thermal resistor”—a tiny component that changes its electrical resistance based on the temperature it senses. On an AO Smith hybrid model, these sensors tell the computer exactly how cold the evaporator coils are or how warm the incoming air is. Here is why they typically fail:

• Thermal Fatigue: Over years of operation, the sensor constantly expands and contracts as it heats and cools. Eventually, the internal wiring can develop microscopic cracks, leading to an “open circuit” that triggers the E13 code.
• Moisture and Corrosion: Because the heat pump creates condensation, the environment inside the top shroud is often humid. If moisture penetrates the sensor’s protective casing, it can corrode the contacts, leading to erratic voltage signals.
• Vibration Displacement: Water heaters vibrate slightly during operation. Over time, these vibrations can wiggle a connector loose from the control board. The computer sees a “missing” sensor and assumes it has failed.
• Voltage Spikes: A power surge in your home can occasionally scramble the sensitive electronics in the thermistor or the control board port it plugs into, causing a permanent communication error.

How to Prevent Error E13

Now that you’ve fixed it, let’s make sure you don’t have to do it again anytime soon!

1. Clean Your Air Filters Regularly: If the air filter on top of your unit is clogged with dust, the heat pump has to work twice as hard, causing the temperatures inside to swing wildly. This puts unnecessary stress on the thermistors. Wash the filter every 3 months with mild soap and water.

2. Install a Whole-House Surge Protector: Since these sensors are sensitive to electrical fluctuations, protecting your home from power surges can extend the life of your water heater’s motherboard and sensors significantly.

3. Manage Humidity: If your water heater is in a very damp basement, consider running a dehumidifier nearby. Reducing the ambient moisture helps prevent the corrosion that often kills these electrical connectors.

Frequently Asked Questions

Q: Can I still use my water heater while the E13 code is active?
A: Generally, yes. Most AO Smith models will default to “Electric Heat Only” mode when a heat pump sensor fails. You will still have hot water, but your unit will be much less energy-efficient until the part is replaced.

Q: Where can I find the specific replacement part number?
A: Look at the data plate on the side of your tank for the model number and serial number. You can call AO Smith technical support or visit their parts website. The thermistor is often sold as a “Sensor Kit” that includes both the ambient and coil sensors.

Q: My multimeter reading was 10k Ohms, but the error persists. What now?
A: If the sensor tests fine, the issue might be the control board itself or a “ghost” error. Try leaving the power off at the breaker for 30 minutes to “cold boot” the system. If the error returns immediately, the control board may need professional diagnosis.