High Lockout
E2 on Worcester Bosch ZWB 7-25

Your boiler has lost the ability to accurately measure the temperature of the water being sent to your radiators, so it has shut down to prevent overheating.

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

CH flow NTC sensor defective.

What causes this fault?

This fault occurs when the boiler's internal thermometer, which monitors the water temperature leaving the heat exchanger, fails to provide a correct reading. The most common reason is that the sensor's internal electrical resistance has drifted out of range due to age, or the connection pins have become corroded, preventing the boiler from operating safely.

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

E9View →EAView →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Radiators cold at the bottom

Sludge build-up — needs flushing, not bleeding.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →