A03
Vokera
Overheat lockout reported by boiler to thermostat
Thermostat shows A03 and heating shuts down unexpectedly
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Blocked or closed radiator valves causing poor circulation, Faulty boiler pump or seized pump reported via bus to thermostat, Air in system causing localised overheating, Incorrect system temperature setting on boiler
How to Fix / Troubleshooting
Safety first: System water can be very hot. Do not open any boiler covers beyond the user-accessible panel. Allow the system to cool before bleeding radiators.
- Check radiators and valves: Ensure all main radiators have their valves open. Partially or fully closed valves can cause the boiler to overheat and send A03 to the thermostat.
- Bleed radiators: With the system off and cool, use a radiator key to bleed trapped air from the highest radiators first. Keep a cloth handy. Top up system pressure at the boiler if it drops below the recommended level.
- Check boiler flow temperature: On the boiler control panel, verify that the CH flow temperature is not set excessively high. For most systems, 65–75°C is adequate. Reduce if necessary.
- Restart system: Reset the boiler if it is locked out, then request heat from the thermostat and monitor whether A03 reappears.
- If A03 recurs frequently: This usually indicates a circulation or boiler internal fault (pump, heat exchanger blockage, sensor). Contact a heating engineer; the thermostat itself is unlikely to be defective.
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Repair Difficulty
Medium
3/5
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