BoilerFaultCodesThe 6 most common faults
Your boiler has shut down because a safety sensor has detected that the internal heat exchanger is getting too hot.
Why it happens: This fault occurs when water cannot travel through the boiler fast enough to take the heat away, causing the internal temperature to spike. The most common reasons are a failed pump that isn't pushing water round or a blockage caused by a buildup of sludge and debris in the system. Small air pockets trapped within the heat exchanger can also trigger this safety shutdown.
Check that your radiator valves (TRVs) are fully open to allow water to flow.Ensure there are no blockages or blankets covering the boiler casing.Press the reset button once to see if the fault clears after the system has cooled.
Your boiler is overheating because the water inside it is getting too hot too quickly, causing the system to shut down for safety.
Why it happens: This fault occurs when the water inside the boiler heats up much faster than it can be moved away, leading to a sudden temperature spike that triggers a safety shutdown. The most common cause is a blockage or a failing pump preventing the water from circulating properly through your radiators. It can also be caused by a buildup of sludge or debris inside the heat exchanger, which restricts the flow of water.
Check that your radiator valves and TRVs are fully openEnsure the boiler pressure is between 1 and 1.5 bar on the gaugeGently bleed your radiators to remove any trapped air
Your boiler has detected that water is leaving the unit much hotter than it is returning, which usually means the water isn't circulating through your radiators fast enough.
Why it happens: This fault occurs when the heat generated by the burner isn't being carried away to your radiators quickly enough, causing heat to build up inside the unit. The most common reason is a failing pump that can no longer push the water around your home, though a blockage caused by sludge or debris in the system can also restrict this flow. If the water stays inside the boiler for too long, it overheats and the system shuts down for safety.
Check that all radiator valves are fully openEnsure the boiler pressure gauge is between 1 and 1.5 barBleed all radiators to remove trapped air that might be blocking water flow
Your boiler has shut down because a sensor has detected that the water inside is getting too hot.
Why it happens: This fault occurs because the water leaving the boiler has exceeded a safe temperature limit, causing the system to shut down for safety. It is usually caused by a lack of water flow, often due to a pump failure or a blockage within the heat exchanger that prevents heat from being carried away quickly enough. In some cases, the sensor itself may simply be faulty and providing an incorrect high-temperature reading.
Check that all radiator valves are fully open to allow water to circulateCheck that your system pressure is between 1.0 and 1.5 bar on the gaugeOnce the boiler has cooled down, try to reset the unit
Your boiler has detected an issue with the electrical power supply to the unit while it was running.
Why it happens: This fault occurs when the boiler detects an unstable electrical signal or a sudden drop in voltage while it is firing. It is usually caused by a loose connection in the internal wiring, a failing control board, or an issue with the electrical circuit providing power to the unit.
Check if other appliances in your home have power.Ensure the boiler's fused spur switch is fully turned on.Reset the boiler by following the instructions on the display panel.
Your boiler has lost communication with your wall thermostat, meaning it cannot receive the signal to turn on your heating.
Why it happens: This fault occurs when the digital communication link between your wall controller and the boiler's internal computer is broken. The most common reason is a wiring failure at the connections or an internal electronic failure within the ModuLine thermostat itself, which prevents the boiler from receiving the command to fire up.
Check if your wall thermostat has run out of batteries and replace them if necessaryEnsure the thermostat is turned up higher than the current room temperatureReset the boiler by turning the control knob to the 'reset' position for five seconds
on the Ideal Maximiser SE
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Ideal shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue