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Worcester Bosch Greenstar 4000 Combi fault 2931

System fault boiler electronics / basic controller (illegal instruction)

Gas boiler

What it means

System detected an illegal instruction in boiler electronics/basic controller.

Likely causes

  1. Faulty burner control unit
  2. Misfiring ignition transformer
  3. Incorrect gas type
  4. Incorrect gas supply pressure
  5. Power supply issue
  6. Faulty electrodes or cable
  7. Damaged or blocked flue system
  8. Incorrect gas/air ratio
  9. Faulty external gas flow monitor (natural gas)
  10. Faulty gas valve
  11. Faulty non-return valve in burner hood assembly
  12. Faulty seals around the burner
  13. Faulty internal flue seals
  14. Faulty case seals
  15. Flue system integrity problem
  16. Combustion circuit problem
  17. Combustion fault
  18. Cleaners containing chlorine or hydrogen halide (spray cans, primers, cleaners, paint, glue) stored or used near the appliance
  19. Build-up of dust

Where to start

  1. Restart appliance
  2. Check the ignition transformer function for misfiring in the Function check menu, Ignition test
  3. Check the gas type

The full answer, free

  • 55 more checks in order
  • The readings and component tests from the manual
  • How to put it right, step by step
  • The manual page for this fault (page 77), right under the answer
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Safety: Isolate gas and power before you open the case. If you have been into the gas side, do a tightness test, and a combustion and flue check before you hand back. Gas Safe (UK) or RGI (Ireland) work only.

From the Worcester Bosch manual for the Greenstar 4000 Combi. Check the model on the data plate before you order parts.

Other Worcester Bosch Greenstar 4000 Combi fault codes