A.O. Smith Tankless E02 Error — What It Means
E02 on an A.O. Smith tankless water heater means the control detected an over-temperature condition. The heated water exceeded the unit’s temperature limit, so the heater shut down into a protective fault state. A.O. Smith’s service documentation states the hardware determined that heated water has exceeded the safe temperature limit. The unit will remain locked out until the cause is corrected and the system is reset.
Common Causes
- Low or no flow through the heater When water does not move properly through the heat exchanger, the unit heats beyond its setpoint or safety limit and triggers the over-temperature fault.
- Failing or incorrect temperature sensor (thermistor) A faulty thermistor can send a false over-temperature reading or prevent the control from regulating heating correctly.
- Loose, damaged, or miswired thermistor wiring Interrupted or corroded sensor connections disrupt the temperature signal and cause the control to trip the fault.
- Cold inlet water below 40°F A.O. Smith specifies that incoming water below 40°F requires the unit to be shut down until the water warms above that threshold.
- Cold-water crossflow or reverse flow Cold water flowing backward through the heater outlet confuses the sensors and can trigger an over-temperature condition.
- Control board issue If sensor and wiring checks do not resolve the fault, the circuit board itself may be defective and require replacement.
Step-by-Step Fix
- Turn off power to the water heater at the breaker and wait 30 seconds to make the unit safe before opening any panels.
- Open a nearby hot-water fixture and let water run through the heater for about 5 minutes to verify flow and see whether the fault clears on its own.
- Check the resistance of each temperature sensor (thermistor) with a multimeter. If one thermistor shows much higher resistance than the others, replace that thermistor.
- Inspect the T-In thermistor wiring and all connections for damage, corrosion, or loose terminals. Repair or replace any faulty wiring or connectors.
- Verify incoming water temperature is above 40°F. If it is below 40°F, close the water supply, drain the heater, and wait until incoming water warms before restoring power.
- Check for reverse flow by feeling the outlet pipe when no fixtures are open. If it is cold or warm when it should be off, install a check valve on the outlet line to prevent cold-water migration.
- Replace the control board if the fault persists after verifying sensors, wiring, and flow conditions. Contact A.O. Smith technical support for board diagnosis and replacement guidance.
Parts Often Needed
| Part | Notes |
|---|---|
| Temperature sensor (thermistor) | Amazon | Replace any sensor that shows resistance much higher than the others or has visible damage. |
| Thermistor wiring harness or connector | Amazon | Use if the sensor wire or plug is corroded, pinched, or broken. |
| Control board (circuit board) | Amazon | Required when sensor and wiring checks are good but the fault will not clear. Verify model compatibility before ordering. |
| Check valve (outlet line) | Amazon | Install on the hot-water outlet if cold-water crossflow or reverse flow is occurring. |
When to Call a Pro
Call a licensed plumber or water-heater technician if you are not comfortable working with 240-volt electrical connections, if you cannot safely access and test the thermistors, or if the fault returns after you have verified flow and checked the sensors. Control board replacement and thermistor testing require a multimeter and familiarity with the unit’s wiring diagram. If incoming water is consistently below 40°F or you suspect a plumbing crossover issue causing reverse flow, a professional can diagnose the supply piping and install the correct backflow prevention.