Reliance water heater displays E4 and will not ignite or heat.

Safety first: Turn OFF power and gas. Allow the unit to cool before handling sensors or piping.

Steps to check:

• 1. Locate inlet sensor: Locate the inlet temperature sensor on the cold water inlet manifold or pipe. Inspect for physical damage or signs of overheating.
• 2. Check wiring and connector: Follow the harness from the sensor to the control board. Look for pinched, cut, or burned wires. Reseat the connector at the board.
• 3. Test sensor: Disconnect the sensor and measure resistance with a multimeter. Compare it to Reliance’s thermistor chart. A reading of 0Ω (short circuit) or infinite (open circuit) indicates a faulty sensor.
• 4. Replace sensor: Install a new Reliance inlet temperature sensor if readings are out of specification. Ensure it is firmly clamped or inserted as designed, and that thermal grease (if originally used) is reapplied.
• 5. Check control board: If a new sensor does not clear the E4 error, the control board input may be defective and require replacement.

Note: Do not interchange inlet and outlet sensors unless the service manual explicitly allows it; they may have different characteristics.