Safety first: Turn off power before working on sensors or wiring.

Step 1 – Locate return sensor: Identify the return (inlet) water temperature sensor on the boiler return piping or heat exchanger.
Step 2 – Check physical mounting: Ensure the sensor is fully inserted into its well or securely clamped to the pipe with thermal paste or insulation as originally installed.
Step 3 – Inspect wiring and connector: Look for loose, corroded, or broken wires. Reseat the connector at both the sensor and the control board.
Step 4 – Measure resistance: With power OFF, disconnect the sensor and measure resistance. Compare to the manufacturer’s chart. Replace the sensor if readings are open, shorted, or far out of range.
Step 5 – Replace control if needed: If the sensor and wiring are good but E6 persists, the control board may be defective and should be replaced by a qualified technician.

Note: Incorrect return temperature readings can cause short cycling and poor system performance.

Boiler displays E6 and may shut down or operate erratically; return temperature reading appears incorrect.

Possible Causes