303 High

Atag 303 Fault Code

The Atag 303 fault code means: the main computer board inside your boiler has developed a memory or communication fault and is unable to operate the system safely.. It appears on 18 Atag models 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: PCB Fault
Typical repair cost £350-£550 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Atag models that show the 303 fault code

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

Source

Atag 303 fault code information on this page is sourced and cross-checked against the official Atag service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026