Common problems guide

The most common Pro boiler problems

The faults we see most often across the Pro range — with plain-English causes, what to check first, estimated repair costs, and whether it's a DIY job or needs a Gas Safe engineer. Based on fault-code data across 4 Pro boiler models.

39 unique codes
4 models covered
8 DIY-safe (top 8)
8 engineer needed

The most common Pro faults

01
low-pressure High DIY-safe

Your boiler has stopped working because the water pressure inside the system has dropped too low to allow it to run safely.

What to try first

Locate the filling loop (usually a silver braided hose with small valves under the boiler).Slowly open the valve(s) until you hear water entering the system and watch the pressure gauge rise.Close the valves tightly once the pressure gauge reaches between 1.0 and 1.5 bar.

Est. cost
£0-160
Parts
Filling loop hose, Expansion vessel, Pressure relief valve
Full guide for A28 low-pressure
02
10 High DIY-safe

Your boiler has failed to ignite, which is usually caused by an issue with the gas flow or the air exhaust system.

What to try first

Check that your gas emergency control valve is in the 'on' positionIf you have a prepaid gas meter, ensure you have sufficient creditVerify that other gas appliances in your home, such as a hob, are working correctlyPress the reset button once for 5 seconds

Est. cost
£100-350
Parts
Gas Valve, Ignition Electrode, Printed Circuit Board (PCB)
Full guide for A28 10
03
AL10 High DIY-safe

Your boiler has stopped working because it cannot light the burner, likely due to a gas supply issue or a blockage in the drainage pipe.

What to try first

Check if your gas supply is active by testing another gas appliance like a hob.Inspect the white plastic pipe under the boiler for any blockages or freezing if the weather is cold.Press the Reset button once to see if the system restarts.

Est. cost
£90-180
Parts
Ignition Electrode, Gas Valve, Condensate Trap
Full guide for A28 AL10
04
AL26 High DIY-safe

Your boiler has sensed that the water returning from your radiators is too hot, which usually means the heat isn't being transferred into your rooms properly.

What to try first

Check that at least 2 or 3 radiator valves (TRVs) are turned to the highest settingEnsure all radiator valves are fully open if you have recently closed themCheck that the bypass valve, if visible, hasn't been accidentally adjusted

Est. cost
£120-280
Parts
Central heating pump, Return thermistor (NTC sensor), External bypass valve
Full guide for A28 AL26
05
AL40 High DIY-safe

Your boiler has stopped working because the water pressure in the central heating system has dropped too low.

What to try first

Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.Slowly open the one or two taps on the loop until you hear water entering the system.Watch the pressure gauge on the boiler until it reaches between 1.0 and 1.5 bar, then close the taps tightly.

Est. cost
£0-120
Parts
Filling loop, Expansion vessel, Pressure sensor
Full guide for A28 AL40
06
AL41 High DIY-safe

Your boiler has stopped working because the water pressure in the system has dropped too low.

What to try first

Locate the filling loop (usually a silver braided hose underneath the boiler).Open the small valve handle(s) slowly to let fresh water into the system.Watch the pressure gauge and close the valves when the needle reaches between 1 and 1.5 bar.

Est. cost
£0-120
Parts
Filling loop, Pressure sensor, Expansion vessel
Full guide for A28 AL41
07
AL74 High DIY-safe

Your boiler has switched itself off because the water pressure is too low, causing the unit to get too hot.

What to try first

Locate the filling loop (the flexible silver hose) underneath your boiler.Slowly open the valves until the pressure gauge needle is between 1.0 and 1.5 bar.Close the valves completely and restart the boiler using the reset button.

Est. cost
£0-120
Parts
None (usually just a manual top-up), Filling loop valve, Expansion vessel
Full guide for A28 AL74
08
AL79 High DIY-safe

Your boiler has reached an unsafe temperature because the water inside cannot move around the system quickly enough to cool down.

What to try first

Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barSwitch the boiler off and back on again to reset the system

Est. cost
£180-450
Parts
Circulation pump, Main heat exchanger, Flow and return thermistors
Full guide for A28 AL79

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