BoilerFaultCodesThe 6 most common faults
Your boiler has shut down because it has overheated or detected a lack of water flow through the system.
Why it happens: This fault occurs when the boiler's internal temperature reaches a dangerously high level, triggering a safety shutdown to prevent damage. The most common reasons are a pump failure that stops water from moving or a buildup of air and sludge that creates a blockage in the heat exchanger. Essentially, the burner is creating heat, but the system cannot move that heat away quickly enough.
Check the pressure gauge and top up water if it is below 1.5 barBleed your radiators to remove trapped airPress the 'Reset' or 'E' button to restart the boiler
Your boiler has shut down because electronic interference or a faulty internal circuit board is preventing the burner from starting safely.
Why it happens: This fault occurs when the boiler's internal computer receives garbled or 'noisy' signals that prevent it from safely managing the burner. It is typically caused by electronic interference from a nearby appliance or a physical defect in the PCB or its unique coding card. Essentially, the boiler has lost its ability to verify its own settings and shuts down as a safety precaution.
Turn off any nearby large electrical appliances or power tools that might cause electronic interference.Press the reset button or turn the boiler off and back on at the fused spur.If the fault persists after a reset, you must call a Gas Safe registered engineer.
Your boiler's internal computer has detected a faulty or damaged temperature sensor, preventing the system from accurately measuring how hot the water is.
Why it happens: This fault occurs when the electrical signal from the temperature sensor is interrupted or compromised, usually due to an internal short circuit. The most common reason is that the sensor's internal components have failed over time or moisture has seeped into the wiring connections. Because the boiler can no longer safely monitor how hot the water is, it shuts down to prevent overheating.
Full guide for 20Your boiler has lost communication with a sensor that monitors the temperature of the water flowing into your heating system, usually because of a loose or damaged wire.
Why it happens: This fault occurs when the boiler's control board can no longer detect a signal from the temperature sensor located on the main pipework. The most common reason is a physical break in the internal wiring or a sensor that has failed due to age, preventing the boiler from knowing how hot the water is as it leaves the unit. Because the system cannot safely monitor the heat, it shuts down to prevent damage.
Full guide for 28Your boiler has stopped working because its internal temperature sensor has developed an electrical fault and can no longer safely monitor the heat level.
Why it happens: This fault occurs when the electrical connection inside the boiler's main temperature sensor fails, causing it to send an incorrect signal to the control board. The most common reason is internal wear or a moisture leak that has damaged the sensor's wiring, making it impossible for the system to safely monitor the water temperature.
Full guide for 30Your boiler has stopped working because it cannot accurately measure its own internal temperature due to a broken or disconnected sensor wire.
Why it happens: This fault occurs when the boiler's main internal thermometer stops sending a steady electrical signal to the control board. The most common reason is a physical break in the thin copper wiring or a connector that has vibrated loose, preventing the boiler from knowing its own temperature. Because the boiler cannot safely monitor if it is overheating, it shuts down the burner as a safety precaution.
Full guide for 38on the Viessmann Vitodens 333
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 Viessmann 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