Malvern TenTwenty Fault codes & diagnostics

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

Regular Natural Gas 3.22-6.15 kW 2000 onwards Discontinued
GC Number 41-555-17
Download the Malvern TenTwenty 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 →

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
41-555-17 tentwenty 2000–present
Fault codes

All 2 documented codes

Tap any card for details

Lockout lamp illuminating (Overheat Thermostat)

Your boiler has switched itself off to prevent damage because the unit has become too hot.

High DIY-safe £90-180

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the system pressure is between 1 and 1.5 bar
  3. Wait for the boiler to cool down and gently press the reset button

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Thermistor
Full explanation for Lockout lamp illuminating (Overheat Thermostat)
Status & diagnostic codes

19 operating states & engineer diagnostics

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

Running lamp flashing Status Your boiler is currently in a normal waiting period and will start back up automatically in a few minutes. 0.5 Diagnostic Your boiler's internal water pressure is outside the required range, meaning it needs a simple top-up to function correctly. 1.0 Diagnostic Your boiler has detected that the water pressure inside the system is too low to operate safely. 1.5 Diagnostic Your boiler's internal water pressure is too low, which is preventing it from circulating heat effectively around your home. 12 Diagnostic Your boiler is displaying a power output reading rather than a fault, which usually indicates the system is in its start-up configuration mode. 14 Diagnostic Your boiler is experiencing a heating output error because the system is failing to correctly regulate its power delivery. 16 Diagnostic Your boiler has detected an issue with the heat output control system and has shut down to prevent the unit from operating outside of its safe temperature limits. 18 Diagnostic Your boiler is struggling to reach its required output, which likely indicates a system pressure issue or a fault with the main sensor. 20 Diagnostic Your boiler is displaying its maximum heat output setting rather than a fault, meaning the system is currently operating at full power. 22 Diagnostic Your boiler has detected an issue with the heat output or ignition process, meaning it has safely shut itself down to prevent damage. 24 Diagnostic Your boiler is currently unable to operate because it is detecting a significant issue with the internal heat output or the way it is communicating with its control system. 26 Diagnostic Your boiler has detected an issue with the water flow or temperature regulation and has safely shut itself down to prevent overheating. 75 mm Diagnostic Your boiler is reporting a physical blockage or debris issue in the flue or drainage pipes, which is preventing it from venting exhaust gases safely. 200 mm Diagnostic Your boiler's flue terminal is positioned too close to the roof overhang, which is preventing exhaust gases from safely dispersing and potentially causing them to be pulled back into the property. 300 Diagnostic Your boiler has lost the water pressure it needs to circulate heat around your home and has automatically shut down for safety. 300 mm Diagnostic Your boiler has detected that it is positioned too close to a window or ventilation point, which poses a safety risk regarding airflow and flue gases. 600 mm Diagnostic Your boiler is indicating that the flue terminal outside your home is obstructed or does not have enough clear space around it to vent safely. 1200 mm Diagnostic Your boiler is warning that the outside flue pipe is dangerously close to an opening like a window or door, which could lead to exhaust fumes entering your home. 1500 mm Diagnostic Your boiler is flagging that the flue terminal is positioned too close to a window or opening, which can be dangerous as it may allow exhaust gases to enter your home.