Worcester Bosch Greenstar 4000 Combi fault 224

What it means

Blocking fault (Category B). Maximal flue gas thermostat or heat exchanger maximal thermostat has activated. If the blocking fault persists for a prolonged period, it becomes a locking fault (also code 224, Category V).

Likely causes

1. Lead disconnected from safety temperature limiter/thermostat or flue thermostat
2. Safety temperature limiter/thermostat or flue thermostat tripped
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
7. Faulty seals around the burner
8. Faulty internal flue seals
9. Faulty case seals
10. Flue system integrity problem
11. Combustion circuit problem
12. Incorrect gas type
13. Incorrect gas supply pressure
14. Damaged or blocked flue system
15. Incorrect gas/air ratio
16. Faulty gas valve
17. Misfiring ignition transformer
18. Power supply issue
19. Faulty electrodes or cable
20. Faulty external gas flow monitor (natural gas)
21. Faulty burner
22. Combustion fault
23. Damaged or insecure flue joints
24. Obstructed or damaged terminal/terminal guard
25. Damaged flue pipes or gaskets
26. Unauthorised modification of flue gas system
27. Leaking flue joints

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 flue thermostat fault).