Baxi [E40] fault codewhat it means and how to fix it

E40 is "Return NTC fault" in the 600, 800, Compact and Duo-tec 2 tables and "Return central heating NTC sensor fault — short circuit/open circuit" on Baxi's professional page. It exists only on the digital-display generation: the selector-switch Duo-tec HE and Platinum have a single heating thermistor and no E40 in their ten-code table. The digital-display boilers added a second sensor on the return into the heat exchanger so the board can compare flow and return (the comparison that produces E154 when it fails), and E40 is that second sensor reading as a broken wire or a short. The boiler stops because it will not run the modulation logic on a missing reading.

Baxi's treatment is the same as for E20: the code carries the spanner symbol, the user guide says a spanner code "cannot be reset by the user" and to contact your installer, annual service provider or Baxi Customer Support, and the engineer's test is the resistance curve, "cold resistance approximately 10 kΩ at 25 °C (CH sensors), resistance reduces with increase in temp, replace sensor".

On the 600 and 800 the return sensor is the same part as the flow sensor, 7207471. Two things distinguish E40 in practice. It is the sensor more often disturbed at a service, because the return pipe is lower and the sensor lead runs past the pump; and an E40 that appears after work on the boiler is more likely a plug not pushed home than a sensor that has failed.