Halstead Best 20 Fault codes & diagnostics

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

Combi Natural Gas 20 kW 2000-2005 Discontinued

This boiler uses indicator light combinations

Note which lights are on, off, or flashing and at what speed. Match the combination to the cards below. Do not reset until you have noted the pattern. How to read fault codes →

Just want the most common Halstead Best 20 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 5 documented codes

Tap any card for details

LED / flash-pattern codes
IGNITION LOCKOUT INDICATOR

Your boiler has stopped working because it failed to light the flame, which usually happens due to a blocked flue, a frozen pipe, or a gas supply issue.

High DIY-safe £120-250

What to check first

  1. Check that your gas supply is turned on at the meter
  2. Ensure any other gas appliances are working to rule out a network supply issue
  3. Press the reset button once and wait for the boiler to cycle

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
  • Flame rectification probe
Full explanation for IGNITION LOCKOUT INDICATOR
OVERHEAT LOCKOUT INDICATOR

Your boiler has switched itself off because it got too hot, which is a safety feature designed to prevent overheating damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Primary pump
  • Diverter valve
  • PCB
Full explanation for OVERHEAT LOCKOUT INDICATOR
Digital codes
Ignition Lock-out neon

Your boiler has stopped working because it failed to light the flame properly, so it has shut itself down for safety.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB
Full explanation for Ignition Lock-out neon
Overheat LED

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

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Central heating pump
  • Plate heat exchanger
Full explanation for Overheat LED
Overheat LED illuminated

Your boiler has detected that it is running too hot and has automatically shut down to prevent damage, meaning you currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • NTC thermistor
  • Pump
  • Plate heat exchanger
Full explanation for Overheat LED illuminated