Worcester Bosch Greenstar RI Heat Only fault codes

6 codes from the Worcester Bosch manual for the Greenstar RI Heat Only gas boiler. Pick one for what it means, the likely causes and where to start.

Code	What it means
1	No ionisation detection after ignition
11	HCM (heat control module) defective
12	Fan fault
2	Loss of ionisation signal during operation
5	Ionisation detected after burner stopped / gas valve EV2 leak test failed / gas valve...
8	MAX safety thermostat activated