BoilerFaultCodes Air Pressure Fault
Air pressure switch not closing, flue blockage, or fan/tube issues.
Worcester Bosch 28 i SI Fault codes & diagnostics
6 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
Combi Natural Gas 28 kW 1999 onwards Discontinued
Replaced by: Greenstar 28i Junior
Download the Worcester Bosch 28 i SI manual
PDF
The fault code is already on your display
Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →
Severity at a glance
Each cell = one fault code. Hover to identify. 6 Unrated
Just want the most common Worcester Bosch 28 i SI faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes
All 6 documented codes
Tap any card for details
Burner Lockout
Failure to ignite, flame sensor issues, or gas valve supply issues.
K
Light indication during fault not covered by other details; control board likely wet or damaged.
L
Gas valve mode switch incorrect, hot water sensor faulty, or gas pressure incorrectly set.
Primary Overheat
Boiler is running in Autofrost stat mode or primary overheat fault.
Sensor Fault
DHW or CH sensor connectivity issues, out of range resistance, or sensor mounting issues.
Status & diagnostic codes 2 operating states & engineer diagnostics
Not faults — these are normal operating states. Tap to expand.
2 operating states & engineer diagnostics
Not faults — these are normal operating states. Tap to expand.