Boiler displays A12, burner output limited, heating slow to reach set temperature

Safety: All gas valve work must be carried out by a licensed gas engineer. Do not attempt to adjust gas settings yourself.

• Observe flame behaviour: If the boiler only runs at low power or fluctuates, modulation may be restricted.
• Check for other gas issues: Confirm that other gas appliances operate normally to rule out supply problems.
• Reset the boiler: Try a reset to clear any temporary control error.
• Call a qualified technician: They will measure modulation signals, check gas pressures, clean or replace the gas valve, and verify combustion with a flue gas analyser.