Baxi · 600 Combi
E321 boiler fault code
Baxi 600 Combi legacy generation
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What E321 means
Domestic hot water temperature (NTC) sensor fault.
Gas Safe engineer: Arrange a Gas Safe registered engineer to investigate this boiler fault.
A boiler fault code alone is not a gas or carbon-monoxide emergency. If you smell gas, a CO alarm sounds, or someone has possible carbon-monoxide symptoms, leave and call 0800 111 999 from outside.
Renting? Report it to your landlord or housing provider first. They are responsible for gas safety and boiler repairs, and the repair should not cost you.
Safe check
Record the displayed code. Do not open the casing or carry out internal checks; arrange a Gas Safe registered engineer.
When to contact an engineer
Arrange a Gas Safe registered engineer to investigate this boiler fault.
Source and provenance
- Baxi Professional error-code table - legacy Baxi 600 Combi & System group - E321 row
professional error-code table
Checked 26/10/2026; the online table publishes no revision