Pro A28 Fault codes & diagnostics

17 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 28 kW 2008 onwards Discontinued
GC Number 47-094-96

Replaced by: A28e

Download the Pro A28 manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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17 High
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-094-96 A28 2008–present
Just want the most common Pro A28 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 17 documented codes

Tap any card for details

10

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

High DIY-safe £100-350

What to check first

  1. Check that your gas emergency control valve is in the 'on' position
  2. If you have a prepaid gas meter, ensure you have sufficient credit
  3. Verify that other gas appliances in your home, such as a hob, are working correctly
  4. Press the reset button once for 5 seconds

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 10
AL10

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.

High DIY-safe £90-180

What to check first

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

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Condensate Trap

Related codes

Full explanation for AL10
AL20

Your boiler has switched itself off because it has detected that an internal component is overheating or getting too hot to operate safely.

High Engineer only £120-£250

Parts commonly replaced

  • Limit thermostat
  • Fumes thermistor
  • Heat exchanger cleaning

Related codes

Full explanation for AL20
AL21

Your boiler has detected an issue with a safety device or an external thermometer, causing it to shut down to prevent overheating.

High Engineer only £90-180

Parts commonly replaced

  • Upper Heat Thermostat (UHT)
  • Condensate Pump
  • External Temperature Sensor

Related codes

Full explanation for AL21
AL26

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.

High DIY-safe £120-280

What to check first

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

Parts commonly replaced

  • Central heating pump
  • Return thermistor (NTC sensor)
  • External bypass valve

Related codes

Full explanation for AL26
AL28

Your boiler has detected that the water coming back from your radiators is hotter than the water going out, which usually means the water isn't flowing through the system correctly.

High Engineer only £120-280

Parts commonly replaced

  • Circulation pump
  • Flow/Return thermistors
  • Main heat exchanger

Related codes

Full explanation for AL28
AL34

Your boiler has stopped working because it cannot correctly detect if the internal fan, which safely clears waste gases, is spinning at the right speed.

High Engineer only £180-£350

Parts commonly replaced

  • Fan Assembly
  • Main Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for AL34
AL40

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

High DIY-safe £0-120

What to check first

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

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for AL40
AL41

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

High DIY-safe £0-120

What to check first

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

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for AL41
AL52

Your boiler's computer brain has encountered an internal software or electronic processing error and has stopped working for safety.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Internal Wiring Loom

Related codes

Full explanation for AL52
AL55

Your boiler's electronic control board has been set up with the wrong configuration settings, preventing it from starting up correctly.

High Engineer only £90-130

Parts commonly replaced

  • Main PCB
  • Jumper Tag
  • Configuration Plug

Related codes

Full explanation for AL55
AL60

Your boiler is struggling to accurately measure the temperature of your hot water, which usually means your taps will only run cold.

High Engineer only £90-160

Parts commonly replaced

  • DHW NTC thermistor
  • Wiring harness

Related codes

Full explanation for AL60
AL71

Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, causing it to shut down as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness / connector

Related codes

Full explanation for AL71
AL73

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning to the unit, causing it to stop running as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Connecting wiring harness

Related codes

Full explanation for AL73
AL74

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

High DIY-safe £0-120

What to check first

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

Parts commonly replaced

  • None (usually just a manual top-up)
  • Filling loop valve
  • Expansion vessel

Related codes

Full explanation for AL74
AL79

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

High DIY-safe £180-450

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Switch the boiler off and back on again to reset the system

Parts commonly replaced

  • Circulation pump
  • Main heat exchanger
  • Flow and return thermistors

Related codes

Full explanation for AL79
low-pressure

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

High DIY-safe £0-160

What to check first

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

Parts commonly replaced

  • Filling loop hose
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for low-pressure