Boiler displays E6; unit may run erratically or not at all

Safety first: Turn off power and allow the boiler to cool before touching sensors or piping.

• Locate the supply sensor: On most Ruud boilers, the supply NTC is mounted on or inserted into the supply outlet pipe or heat exchanger manifold.
• Inspect wiring: Check the sensor leads for cuts, burns, or pulled‑out connectors. Reseat the plug at the control board.
• Check for corrosion or moisture: If the sensor is in a well, look for signs of leakage around the well. Any water intrusion can damage the sensor and should be corrected.
• Test resistance (if equipped): With power off and wires disconnected, measure the sensor resistance with a multimeter and compare to the Ruud temperature/resistance chart. Infinite or zero ohms indicates a bad sensor.
• Replace if faulty: Remove the old sensor (some are clipped on, others threaded into a well) and install the new Ruud‑specified NTC. Restore wiring and power, then clear the error.