Reliance tankless or power vent water heater displays E3 and shuts down during operation.

Safety first: Turn OFF power and gas. Do not operate the heater with a suspected vent blockage; there is a risk of carbon monoxide exposure.

Steps to check:

• 1. Inspect vent termination: Go outside and inspect the vent and intake terminations. Remove any debris, nests, snow, or vegetation blocking airflow.
• 2. Check venting layout: Verify that the vent is installed according to Reliance specifications (correct diameter, length, number of elbows, and slope). Excessive length or improper slope can cause overheating.
• 3. Inspect intake air path: For direct-vent models, ensure the intake pipe is not crushed or blocked. For indoor combustion air, verify the room has adequate ventilation.
• 4. Examine flue temperature sensor: With power OFF, locate the flue/exhaust temperature sensor on the heat exchanger or vent collar. Check wiring and connector. If the wiring is intact, test the sensor resistance and compare it to the specifications; replace if out of range.
• 5. Descale heat exchanger: Heavy scale can cause hot spots and trigger the E3 error. Perform a full descaling procedure using service valves, a pump, and descaling solution.

When to call a professional: If the venting appears incorrectly installed or you suspect combustion issues, have a licensed technician evaluate and correct the vent system.