Common problems guide

The most common Worcester Bosch 28 Si RSF problems

The faults most likely to send a Worcester Bosch 28 Si RSF into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.

The 6 most common faults

01

Lock-out High Engineer

Your boiler is failing to light the flame properly, so it has purposefully shut down to prevent gas from building up inside.

Est. cost: £150-300
Parts: Ignition electrode, Gas valve, Ignition lead

Full guide for Lock-out

02

CH light On High Engineer

Your boiler has automatically turned itself off because the water inside it is getting too hot, likely due to a circulation problem.

Est. cost: £120-250
Parts: Pump, Heat exchanger, NTC sensor

Full guide for CH light On

03

FAIL POINT K High Engineer

Your boiler is failing to light the gas burner because it is not receiving the correct signal to start the heating process.

Est. cost: £120-250
Parts: Gas valve, Wiring harness, PCB

Full guide for FAIL POINT K

04

Slow flash High Engineer

Your boiler has detected a safety concern or internal fault and has automatically shut down to protect itself, requiring you to attempt a manual reset.

Est. cost: £120-250
Parts: Printed circuit board, Ignition electrode, Flame rectification probe

Full guide for Slow flash

05

Fast flash High Engineer

Your boiler has detected an internal safety lockout error, causing it to stop operating to prevent potential damage or overheating.

Est. cost: £150-300
Parts: Printed Circuit Board (PCB), Air pressure switch, Flue fan

Full guide for Fast flash

06

2 lights flashing fast High Engineer

Your boiler has detected an internal fault preventing it from lighting, most likely caused by a failed safety sensor or a malfunction in the ignition sequence.

Est. cost: £150-300
Parts: Air pressure switch, Flue fan, Printed Circuit Board (PCB)

Full guide for 2 lights flashing fast

Parts most often replaced

on the Worcester Bosch 28 Si RSF

Ignition electrodeGas valveIgnition leadPumpHeat exchangerNTC sensor

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

You can smell gas or see signs of a leak
The Worcester Bosch shows an Emergency or High severity code
The boiler keeps locking out after repeated resets
You've tried the DIY checks and the fault hasn't cleared
There's visible water leaking from the boiler
The flame is yellow or orange instead of blue

See the full 8-code list for the Worcester Bosch 28 Si RSF