A-A5 High

Worcester Bosch A-A5 Fault Code

Also written as AA5 — same fault.

The Worcester Bosch A-A5 fault code means: your boiler has detected that the water temperature has risen too quickly or reached an unusually high level, causing it to shut down to prevent overheating.. It appears on 1 Worcester Bosch model in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Actual temperature: Flow temperature at the flow temperature sensor

What causes the Worcester Bosch A-A5 fault?

This fault occurs when water cannot travel through the boiler’s heat exchanger fast enough to carry the heat away, causing a rapid temperature spike. The most common reasons are a failed internal pump, a blockage caused by sludge or debris in the pipework, or air trapped within the system preventing proper circulation.

This issue often surfaces during the first cold spell of autumn when the heating is turned on after being idle all summer, which can cause internal pump parts to seize.

Typical repair cost £120-350 (including labour)

DIY fix possible

Check that all radiator valves are fully open to allow water to flow.Ensure the system pressure gauge is between 1.0 and 1.5 bar.Try resetting the boiler via the display panel to see if the fault clears.

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Worcester Bosch models that show the A-A5 fault code

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

Source

Worcester Bosch A-A5 fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.

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