Grant Outdoor Combi 90 Mkii Fault codes & diagnostics

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

Combi Oil 26 kW 2004-2009 Discontinued

Replaced by: Vortex Outdoor Combi

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
6 High
Just want the most common Grant Outdoor Combi 90 Mkii faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 6 documented codes

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Amber neon

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

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a flexible silver hose) under the boiler or in the airing cupboard.
  2. Open the small valve black handles slowly until you hear water entering the system.
  3. Watch the pressure gauge and close the valves when it reaches 1.5 bar, then press the reset button on the control panel.

Parts commonly replaced

  • Automatic Air Vent
  • Expansion Vessel
  • Pressure Relief Valve
Full explanation for Amber neon
Amber neon (on panel)

Your boiler has detected that the water pressure in the system is too low to run safely and has switched itself off to prevent damage.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (a flexible silver hose) usually found under the boiler or near the hot water cylinder.
  2. Open the small black valves slowly until you hear water entering the system.
  3. Watch the pressure gauge and close the valves once the needle reaches between 1 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve
Full explanation for Amber neon (on panel)
Lock-out

Your boiler has stopped because it failed to ignite, so you need to perform a simple reset to try and wake it up again.

High DIY-safe £120-250

What to check first

  1. Ensure your heating oil tank has sufficient fuel
  2. Wait 45 seconds for the boiler to cycle internally
  3. Press the illuminated red reset button once to restart the burner

Parts commonly replaced

  • Photocell
  • Ignition electrodes
  • Oil burner nozzle
  • Control box
Full explanation for Lock-out
Overheat thermostat

Your boiler has become too hot and has automatically shut itself down to prevent damage or safety issues.

High DIY-safe £90-180

What to check first

  1. Identify the black reset button cover on the control panel
  2. Unscrew the cover and firmly press the small reset button underneath until it clicks
  3. Ensure your radiator valves are open and the system pressure is correct

Parts commonly replaced

  • Overheat Limit Thermostat
  • Circulation Pump
  • Thermistor
Full explanation for Overheat thermostat
Red neon

Your boiler has switched itself off because it has reached an internal temperature that is too high, preventing potential damage or safety issues.

High DIY-safe £80-200

What to check first

  1. Wait for the boiler to cool down completely (at least 20-30 minutes).
  2. Locate the overheat reset button under the screw-on cap on the control panel.
  3. Press the button firmly until you hear a click, then restart the boiler.

Parts commonly replaced

  • Control Thermostat
  • Safety Overheat Thermostat
  • Circulating Pump
Full explanation for Red neon
Red neon (on panel)

Your boiler has shut itself down because it became too hot, acting as a safety precaution to prevent damage.

High DIY-safe £90-220

What to check first

  1. Locate the reset button on the control panel, which may be hidden behind a small screw-off plastic cap.
  2. Ensure all radiator valves are open and the system pressure is between 1 and 1.5 bar on the gauge.
  3. Press the reset button firmly; if the boiler restarts and runs, monitor it, but call a technician if it happens again.

Parts commonly replaced

  • Overheat Thermostat
  • Circulating Pump
  • System Pressure Sensor
Full explanation for Red neon (on panel)