Heater shows E06 and will not start; sometimes appears intermittently with temperature fluctuations.

Safety first: Disconnect power to the heater before working on electrical components.

Step-by-step checks:

• Locate inlet sensor: Find the inlet temperature sensor on the heater's cold-water inlet header or pipe. Ensure it is properly seated and secured.
• Inspect cable and plug: Follow the cable to the main control board. Look for signs of rodent damage, UV cracking, or moisture ingress at the connector. Dry any moisture and reseat the plug.
• Test sensor resistance: With the sensor unplugged and power off, measure its resistance and compare to the Pahlen chart for the current water temperature. Replace the sensor if the reading is open, shorted, or far from specification.
• Swap sensors (if identical): On some models, inlet and outlet sensors are identical. Temporarily swap them to see if the error code follows the sensor. If it does, replace the faulty sensor.

When to stop: If both sensors test correctly and wiring is intact, the control board may be defective and should be diagnosed by a professional.