How much does it cost to fix Baxi error code 433?

Typical repair cost for the Baxi 433 fault is £120-£350 including labour.

433 High

Baxi 433 Fault Code

Q: What does the Baxi 433 fault code mean?

Your boiler has shut down because it has detected that it is getting far too hot, likely due to water not circulating properly through the system.

The Baxi 433 fault code means: your boiler has shut down because it has detected that it is getting far too hot, likely due to water not circulating properly through the system.. It appears on 26 Baxi models in our database.

Technical description: Heat exchanger temperature exceeded (greater than 95 degrees)

What causes the Baxi 433 fault?

This fault occurs when the water inside the boiler reaches a dangerously high temperature because it isn't moving away from the heat source quickly enough. The most common reason is a failed pump or a blockage in the system, such as a build-up of sludge or a stuck valve, which prevents the hot water from circulating to your radiators.

This issue often surfaces when the heating is first turned on after summer, as the internal pump or diverter valve can seize up during long periods of inactivity.

Typical repair cost £120-£350 (including labour)

DIY fix possible

Check that your radiator valves are fully openEnsure there is enough water pressure in the system (usually between 1 and 1.5 bar)Bleed your radiators to remove any trapped airReset the boiler using the selector switch

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Baxi models that show the 433 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Baxi 433 fault code information on this page is sourced and cross-checked against the official Baxi service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026