A.O. Smith E21 Error Code — What It Means
The E21 error code on an A.O. Smith electric tankless water heater indicates a shorted thermistor. On 2-chamber and 4-chamber models, the fault points to TH-1. On point-of-use models, it points to the T-Out sensor. A short means the thermistor circuit is reading an abnormally low resistance, usually because the sensor element itself has failed internally or because damaged wiring is creating a false short condition. This is different from an open-circuit thermistor fault. The control board cannot read accurate temperature and shuts down the unit for safety.
Common Causes
- Failed thermistor element The sensor has electrically shorted internally and reads zero or near-zero resistance instead of the normal temperature-dependent value.
- Damaged or pinched thermistor wiring Pinched, cut, or corroded wiring in the thermistor circuit creates an unintended short to ground or between conductors.
- Wet or contaminated sensor connector Moisture, mineral deposits, or corrosion on the thermistor plug or socket can create a false short reading at the connection point.
- Loose or corroded thermistor connector Poor contact at the sensor plug can produce intermittent short readings that trigger E21.
- Faulty control board input circuit If the thermistor and wiring test normal but the code persists, the control board itself may have a shorted input circuit that needs replacement.
Step-by-Step Fix
- Shut off power at the circuit breaker before servicing the heater.
- Restore power and attempt a restart to verify the E21 code persists after a normal power cycle.
- Power off again, then inspect the thermistor wiring and connector for visible damage, pinching, corrosion, or moisture.
- Disconnect and reconnect the thermistor plug at both the sensor and control board ends to confirm a clean connection, then restart and retest.
- Measure the thermistor resistance with power off using a multimeter, and compare the reading to any other accessible thermistors on the unit. If one sensor is much lower in resistance than the others or reads near zero ohms, it is shorted.
- Replace the shorted thermistor (TH-1 on 2- and 4-chamber models, T-Out on point-of-use models) with the manufacturer-specified replacement sensor for your model.
- If the E21 code returns after thermistor replacement, replace the control board or contact A.O. Smith technical support for additional diagnostics.
Parts Often Needed
| Part | Notes |
|---|---|
| TH-1 thermistor (2-chamber and 4-chamber models) | Amazon | Verify your model number and chamber count before ordering the correct inlet thermistor. |
| T-Out thermistor (point-of-use models) | Amazon | Outlet temperature sensor for single-point units. Confirm part number from your model’s service label. |
| Control board / circuit board | Amazon | Required only if the thermistor and wiring test normal but E21 persists after sensor replacement. |
When to Call a Pro
Call a licensed plumber or electrician if you are not comfortable working with 240 V electric water heaters or if you lack a multimeter to test thermistor resistance. If you have replaced the thermistor and verified all wiring but the E21 code still appears, the control board is likely at fault and requires professional diagnosis and replacement. Some jurisdictions require permitted electrical work for control board replacement on permanently wired appliances.