Safety first: Turn OFF power at the breaker and close the gas valve before removing covers.

Step 1 – Visual inspection: Remove the front cover. Locate the inlet and outlet thermistors on the water piping near the heat exchanger. Inspect for broken wires, loose connectors, or signs of overheating.
Step 2 – Reseat connectors: Disconnect and reconnect each thermistor plug to ensure a solid connection. Look for corrosion on pins.
Step 3 – Basic resistance check (for advanced DIYers): With power OFF and wires disconnected, measure resistance across each thermistor with a multimeter. Compare readings to Ruud service specifications for the current water temperature. A reading of 0 Ω (short) or infinite (open) indicates a bad sensor.
Step 4 – Replace faulty thermistor: Shut off water if needed. Remove the retaining clip or screw, pull out the thermistor, and install a new OEM Ruud thermistor of the same type (inlet or outlet). Reconnect wiring.
Step 5 – Test operation: Restore power and gas, run hot water, and confirm the E4 code is cleared.

If both sensors test good and wiring is intact, the control board may be defective and should be evaluated by a professional.

Tankless water heater displays E4; unit may not start or may shut down during operation.

Possible Causes