Alpha Cb50 Fault codes & diagnostics

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

Combi Natural Gas 50 kW 2007-2016 Discontinued

Replaced by: E-Tec Plus 38

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 10 High 1 Medium

All 12 documented codes

Tap any card for details

A Flashing

Your boiler's internal thermometer is failing to send reliable signals, so the system has shut down to prevent it from overheating.

High Engineer only £120-220

Parts commonly replaced

  • NTC Thermistor
  • PCB

Related codes

B Flashing

Your boiler has detected that it is getting too hot and has automatically shut down to prevent any damage or safety hazards.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Diverter valve
  • Heat exchanger
B Illuminated continuously

Your boiler has stopped working because it failed to ignite the gas burner, meaning you will currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB
E01

Your boiler has failed to light the flame needed to heat your water and radiators.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

E02

Your boiler has detected that it is running too hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Diverter valve
  • Primary heat exchanger

Related codes

E03

Your boiler has detected an issue with the airflow or ventilation system, and for safety reasons, it has turned itself off to prevent harmful gases from escaping.

Emergency Engineer only £150-300

Parts commonly replaced

  • Air pressure switch
  • Flue fan unit
  • Wiring harness

Related codes

E05

Your boiler has lost the ability to accurately measure its own water temperature, so it has safely shut down to prevent overheating.

High Engineer only £120-180

Parts commonly replaced

  • NTC Sensor
  • Wiring harness

Related codes

E06

Your boiler is struggling to accurately measure the temperature of the hot water it produces, so it has turned off the hot water supply to prevent overheating.

High Engineer only £120-200

Parts commonly replaced

  • DHW NTC sensor
  • Wiring harness

Related codes

E10

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

High DIY-safe £80-150

What to check first

  1. Locate the external filling loop, which is usually a flexible silver braided hose with one or two valves located underneath the boiler.
  2. Open the valve(s) slowly to allow mains water into the heating system while watching the pressure gauge on the front of the boiler.
  3. Close the valve(s) tightly once the needle on the gauge reaches the green zone, typically between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor
E16

Your boiler has stopped working because the internal component responsible for pushing exhaust gases out of the flue is not operating correctly.

High Engineer only £180-350

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Printed Circuit Board (PCB)
E24

Your boiler is struggling because it detects a temperature difference between the water leaving and returning that should not be there, causing it to shut down for safety.

High Engineer only £150-250

Parts commonly replaced

  • NTC thermistors
  • Primary heat exchanger
  • Pump

Related codes

E31

Your boiler has lost its connection with the digital thermostat or timer controller, meaning it cannot receive instructions to heat your home or water.

Medium DIY-safe £120-250

What to check first

  1. Switch off the power to the boiler at the fused spur for 60 seconds
  2. Turn the power back on and see if the connection is re-established
  3. Check that the batteries in your remote thermostat controller are fresh and fully charged

Parts commonly replaced

  • Remote control receiver
  • PCB (Printed Circuit Board)
  • Communication interface cable

Related codes