High Lockout
E16 on Viessmann Vitodens 100-W

Your boiler has stopped heating because a safety sensor that monitors exhaust gases has detected an electrical fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Burner does not start. Flue gas sensor fault (short circuit or lead break).

What causes this fault?

This fault occurs when the boiler's internal computer stops receiving a reliable signal from the sensor that monitors the temperature of the exhaust gases. It is usually caused by the sensor itself failing internally or the electrical wiring connecting it to the main control board becoming damaged or loose. Because this sensor is a vital safety component, the boiler stops immediately to prevent the system from overheating.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

F2View →F3View →A9View →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →