Baxi [E321] fault codewhat it means and how to fix it

E321 is "DHW NTC fault" in the 600, 800, Compact and Duo-tec 2 tables, and Baxi's professional page lists it twice as "E321 / 163 — Domestic Hot Water NTC sensor fault (s.c.)" and "(o.c.)", short and open circuit, the "163" being the alternative number some of these boards show. It is the digital-display generation's number for the fault the selector-switch Duo-tec HE and Platinum call E50: the thermistor on the hot water side of the plate heat exchanger reading as a broken wire or a short.

Without it the board cannot hold the tap water at the set temperature, so it stops the hot water function and shows the code, while on most of these combis the heating carries on. E321 shows with the spanner symbol, so the user guide's instruction applies — a spanner code "cannot be reset by the user. Please contact your Installer, Annual Service Provider or Baxi Customer Support" — and there is no useful reset anyway, since the code returns the moment the board reads the sensor.

The engineer's test is the same resistance curve Baxi print for the other thermistors, and the DHW sensor on the 600 and 800 sits on the hydraulic outlet manifold, reached once the case front is off. A caution on symptoms: hot water that runs scalding or cold with no code is more often a diverter valve or a scaled plate exchanger; E321 is specifically the sensor circuit, and Baxi show a code for it precisely so the two are not confused.