Worcester Bosch Greenstar 4000 System fault 224

What it means

Blocking fault (B category) that becomes a locking fault (V category) if it persists for a prolonged period. Indicates the maximal flue gas thermostat or heat exchanger maximal thermostat has tripped due to overtemperature conditions.

Likely causes

1. Disconnected lead from safety temperature limiter/thermostat or flue thermostat
2. Faulty high limit safety temperature limiter/thermostat or flue thermostat
3. Low system pressure
4. Air trapped in the system or boiler
5. Pump seized or not running
6. Blockage/restriction within boiler/pipework restricting flow

Where to start

1. Check if the lead is disconnected from the safety temperature limiter/thermostat or the flue thermostat.
2. Check the high limit of the safety temperature limiter/thermostat or the flue thermostat and the harness for continuity.
3. Check the flow temperature to confirm actual temperature (may have tripped out at lower temperature, leading to a safety temperature limiter/thermostat or the flue thermostat fault).