Common problems guide

The most common Worcester Bosch 28 i SI problems

The faults most likely to send a Worcester Bosch 28 i SI 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

Overheat Cut-off High Engineer

Your boiler has switched itself off because it has detected an unsafe rise in internal temperature and needs to cool down before it can restart.

Est. cost: £150-300
Parts: NTC temperature sensor, Central heating pump, Primary heat exchanger

Full guide for Overheat Cut-off

02

Ignition Lockout High Engineer

Your boiler has failed to start the flame needed to heat your home and has automatically locked itself for safety to prevent a gas buildup.

Est. cost: £150-350
Parts: Ignition electrode, Gas valve, Printed Circuit Board

Full guide for Ignition Lockout

03

Air Flow Device High Engineer

Your boiler is struggling to clear exhaust gases safely, so it has stopped working to prevent any risk to your home.

Est. cost: £150-300
Parts: Fan assembly, Pressure switch, Flue seals

Full guide for Air Flow Device

04

Lock-out High Engineer

Your boiler is failing to ignite the flame it needs to heat your water, so it has safely shut itself down to prevent a build-up of unburnt gas.

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

Full guide for Lock-out

05

CH light On High Engineer

Your boiler has automatically turned itself off because it is getting too hot, likely due to a flow issue within your heating system.

Est. cost: £120-250
Parts: Central Heating Pump, Plate Heat Exchanger, NTC Sensor

Full guide for CH light On

06

FAIL POINT K High Engineer

Your boiler is struggling to detect gas or initiate the heating process, which may be caused by a loose internal connection or an issue with your gas supply.

Est. cost: £150-250
Parts: Wiring harness, Gas valve

Full guide for FAIL POINT K

Parts most often replaced

on the Worcester Bosch 28 i SI

NTC temperature sensorCentral heating pumpPrimary heat exchangerIgnition electrodeGas valvePrinted Circuit Board

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 11-code list for the Worcester Bosch 28 i SI