Halstead Ace High Fault codes & diagnostics

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

Regular Natural Gas 20 kW 2006-2010 Discontinued
Download the Halstead Ace High manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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 →

Just want the most common Halstead Ace High 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
Power ON LED (Green) blinking

Your boiler has detected that there is either not enough water inside the heating system or the sensor responsible for detecting flow is not operating correctly.

High DIY-safe £120-220

What to check first

  1. Check your boiler's pressure gauge and ensure it is between 1 and 1.5 bar.
  2. If the pressure is low, locate the filling loop under the boiler to top up the system.
  3. Once the pressure is correct, restart the boiler to see if the blinking light stops.

Parts commonly replaced

  • Diverter valve microswitch
  • Flow switch
  • Pressure sensor
Full explanation for Power ON LED (Green) blinking
Digital codes
Ignition Lock-out

Your boiler has stopped working because it cannot light the flame required to produce heat or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB
Full explanation for Ignition Lock-out
OVERHEAT THERMOSTAT

Your boiler has detected that it is getting too hot and has automatically switched itself off to prevent any damage or safety issues.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Diverter valve
  • System filter
Full explanation for OVERHEAT THERMOSTAT
RED LED ON

Your boiler has stopped working because it has failed to start properly or has become too hot and turned itself off as a safety precaution.

High Engineer only £120-250

Parts commonly replaced

  • Ignition Electrodes
  • Overheat Thermostat
  • Printed Circuit Board
Full explanation for RED LED ON
Safety Lockout LED (Red)

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

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB
Full explanation for Safety Lockout LED (Red)
Status & diagnostic codes

8 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

GREEN LED FLASHING Status Your boiler is perfectly healthy and is simply waiting for a signal from your thermostat or hot water tap to start heating. Burner ON LED (Green) Status Your boiler is operating normally and is currently heating your water or home. C12 Diagnostic Your boiler has lost its internal settings, meaning it no longer knows how to correctly identify or operate its own hardware. C32 Diagnostic Your boiler has lost its internal configuration settings and no longer recognises how it should operate. GREEN LED ON Status Your boiler is working exactly as it should and is currently fulfilling a request for heating or hot water. Heating LED (Green) Status Your boiler is operating exactly as it should and is currently providing heat to your radiators. Hot Water LED (Green) Status Your boiler is currently operating normally as it heats the water for your taps and shower. Power ON LED (Green) Status Your boiler is working perfectly fine and is simply waiting for you to turn on a tap or adjust your heating settings.