Baxi [E.04.01] fault codewhat it means and how to fix it

Baxi's newer displays give the flow sensor two codes where the older Baxis gave it one. E.04.01 is "flow temperature sensor short circuited" and E.04.02 "flow temperature sensor disconnected" (the 200/400 manual says "open-circuited"), and together they are what E20 is on the three-digit ranges: the thermistor on the flow out of the heat exchanger reading as a short or a broken wire, so the board has no water temperature to work from and stops. Having two codes tells the engineer which way the circuit has failed before the case is off. A short is usually water in the connector or a sensor that has failed internally; an open is usually a plug pulled or a lead broken.

Baxi's remedy is the same for both and is printed in full on the 200/400: "sensor not or badly connected, bad connection, sensor fault — check the wiring, check that the sensors are operating correctly, check that the sensor has been correctly fitted, replace the sensor if necessary". The Combi 2 table condenses it to "SENSOR/CONNECTION PROBLEM — check the sensor/PCB connection, check the operation of the sensor". The user manual for the 200/400 lists E04 01 and E04 02 together as "flow temperature sensor fault" and, like every permanent fault on these boilers, the reset is the RESET button, "for at least 3 seconds" there and "for one second" on the Combi 2.

In practice the reset achieves nothing until the circuit is repaired, since the board re-reads the sensor at once. The same two-code pattern covers the other sensors on these ranges: E.00.04 and E.00.05 for the return sensor, E.04.04 and E.04.05 (200/400) or E.00.20 and E.00.21 (Combi 2) for the flue sensor, and E.00.16 and E.00.17 for a cylinder sensor on the System 2. Each has its own alias here but the fault and the remedy are the same sentence.