What does A.O. Smith Tankless E20 Error - Causes & Fix mean?

E20 means the inlet thermistor is shorted. The fix is to replace the inlet temperature sensor or check its wiring and connections.

How do I fix A.O. Smith Tankless E20 Error - Causes & Fix?

Follow the step-by-step diagnosis below. The most likely fix is identified by checking power, sensor resistance, and control-board signals in order — using a multimeter and the part numbers documented in the parts reference table.

Can I keep running the equipment with E20 active?

Generally no. Faults on Water heater units are emitted when a sensor, safety, or control circuit is outside its operating envelope. Running through an active fault risks compressor, heater, or control-board damage. Diagnose and clear the fault first.

What part is most commonly replaced for A.O. Smith Tankless E20 Error - Causes & Fix?

The most frequently replaced parts for A.O. Smith Tankless E20 Error - Causes & Fix are listed in the Parts Reference section of this guide with OEM part numbers and typical price ranges. Check the table for direct replacements.

Is A.O. Smith Tankless E20 Error - Causes & Fix a DIY repair or do I need a technician?

Sensor swaps, fan motor replacements, and condenser cleaning are typical DIY-level repairs. Anything involving refrigerant, gas, or high-voltage (>240V) work requires a licensed technician for both safety and code-compliance reasons.

A.O. Smith Tankless E20 Error - Causes & Fix

A.O. Smith Tankless E20 Error — What It Means

The E20 code on an A.O. Smith electric tankless water heater signals that the inlet thermistor (temperature sensor) has shorted. The manufacturer’s service documentation ties this fault directly to the inlet sensor and directs technicians to replace it rather than attempt a simple reset.

This is not a general overheat or flow issue. It is a specific electrical fault inside the sensor or its wiring. The heater will not operate normally until the shorted component is identified and replaced.

Jump to Fix

Common Causes

Failed inlet thermistor An internal short inside the sensor is the direct manufacturer-listed cause for E20.
Damaged sensor harness or connector A pinched wire, corroded pin, or loose plug at the inlet thermistor can mimic a shorted sensor.
Poor electrical connection Oxidation or vibration can create intermittent contact that reads as a short to the control board.
Abnormal thermistor resistance If measured resistance is far outside the normal range compared to other sensors, the thermistor has failed.
Faulty control board If the code persists after sensor replacement and wiring checks, the circuit board itself may be misreading or defective.

Step-by-Step Fix

Turn off all power to the unit at the circuit breaker and confirm it is de-energized before opening any panel or touching wiring.
Run hot water through the heater by opening a nearby fixture for about five minutes to stabilize operating conditions for testing.
Locate the inlet thermistor on the inlet water line and inspect its connector and wiring for damage, corrosion, or pinched insulation.
Disconnect and reconnect the thermistor plug firmly to rule out a poor connection, then clear the code and test.
Measure thermistor resistance if you have a multimeter and compare it to the other sensors or consult your model’s service table for acceptable values.
Replace the inlet temperature sensor if resistance is abnormal, the connector shows damage, or the code returns immediately after clearing.
Replace the control board if the E20 fault persists after installing a new inlet thermistor and verifying all connections are secure.

Parts Often Needed

Part	Notes
Inlet thermistor / inlet temperature sensor	Amazon \| Primary replacement part for E20. Match the sensor to your model number.
Control board / circuit board	Amazon \| Required if the fault remains after sensor replacement and all wiring checks pass.

When to Call a Pro

Call a licensed technician if you are not comfortable working with live electrical circuits or if you lack a multimeter to verify thermistor resistance. Also contact a pro if the error returns after you have replaced the inlet sensor and inspected all connections. Persistent E20 codes after sensor replacement usually point to a control board fault, and the manufacturer recommends technical support for board-level diagnosis.