Thermostat shows E04 and cannot detect the external wired sensor (e.g. remote room or floor sensor).

Safety first: Turn off power to the thermostat and boiler before working on low-voltage sensor wiring.

Steps to diagnose:

• 1. Confirm sensor configuration: In the thermostat settings, verify that an external sensor is enabled only if one is actually installed. Disable it if not used to clear the error.
• 2. Inspect wiring: Locate the external sensor cable at the thermostat and at the sensor end. Check for loose terminals, damaged insulation, or crushed cable.
• 3. Test sensor resistance: With power off, disconnect the sensor wires from the thermostat. Use a multimeter to measure resistance across the sensor. A typical NTC sensor will show a resistance in the kilo-ohm range (refer to Baxi sensor chart). Infinite or near-zero resistance indicates a fault.
• 4. Check cable continuity: If the sensor tests good at its end, test continuity of each conductor from thermostat to sensor to rule out cable breaks.

When to call a professional: If wiring is intact but the thermostat still reports E04 with a known-good Baxi sensor, the thermostat’s sensor input circuit may be damaged and require replacement.