Baxi / Main / Potterton · Compact, Duo-tec, Platinum, Assure and EcoBlue platform
E321 / 163DHW NTC sensor open- or short-circuit
This code applies to Baxi Platinum Compact Combi, Baxi Duo-tec Compact Combi, Baxi Megaflo Compact System, Main Eco Compact Combi & System, Baxi 600 Combi & System, Baxi 800 Combi & System, Baxi Duo-tec Combi, Baxi Assure Combi & System, Baxi Platinum Combi, Potterton Assure Combi & System, Baxi Platinum+ Combi, Baxi Megaflo System, Baxi EcoBlue Combi & System. Fault codes narrow down the likely problem, but they do not replace a model-specific diagnosis.
What you can safely check
- No internal homeowner repair is safe for this fault.
- A single reset may be tried only where the model's user manual allows it.
- Arrange an engineer if the code remains or returns.
How an engineer may fix it
Test the relevant sensor or thermistor, its position, resistance, plug and wiring; repair the connection or replace the failed sensor and verify operation.
Manufacturer source: Baxi Professional — Boiler Error Codes. Checked 15 August 2026. The appliance manual and an engineer’s on-site diagnosis take precedence. Never remove the boiler case yourself.