Common problems guide

The most common Baxi Combi 224, 228, 424, 428 problems

The faults most likely to send a Baxi Combi 224, 228, 424, 428 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

H:01 .00 High Engineer

Your boiler has lost its connection to the fuel control system, meaning it cannot safely ignite to provide heating or hot water.

Est. cost: £150-300
Parts: Gas valve wiring harness, Gas valve, Main printed circuit board (PCB)

Full guide for H:01 .00

02

H:01 .05 High Engineer

Your boiler has detected that the water leaving it is significantly hotter than the water returning, suggesting a blockage or a problem with how water is circulating around your heating system.

Est. cost: £150-300
Parts: Central Heating Pump, Plate Heat Exchanger, Diverter Valve

Full guide for H:01 .05

03

H:01 .08 High Engineer

Your boiler is shutting down because it detects that the water inside is heating up much faster than it safely should.

Est. cost: £150-300
Parts: Central heating pump, Flow and return thermistors, Plate heat exchanger

Full guide for H:01 .08

04

H:01 .14 High Engineer

Your boiler has detected that the water inside the unit has become dangerously hot and has shut itself down to prevent damage.

Est. cost: £120-250
Parts: NTC Sensor, Primary Heat Exchanger, Pump

Full guide for H:01 .14

05

H:01 .18 High Engineer

Your boiler is failing to move hot water around your radiators or taps, likely because it detects a blockage or a pump issue that prevents it from running safely.

Est. cost: £150-350
Parts: Central heating pump, Diverter valve, System filter

Full guide for H:01 .18

06

H:01 .21 High Engineer

Your boiler is struggling to control the temperature of your hot water, causing it to heat up much faster than it safely should.

Est. cost: £150-250
Parts: DHW flow sensor, DHW temperature sensor (NTC), Diverter valve

Full guide for H:01 .21

Parts most often replaced

on the Baxi Combi 224, 228, 424, 428

Gas valve wiring harnessGas valveMain printed circuit board (PCB)Central Heating PumpPlate Heat ExchangerDiverter Valve

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 Baxi 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 28-code list for the Baxi Combi 224, 228, 424, 428