Emergency
E773 on Navien NCB-40LDWE

Your boiler has detected that the type of gas being supplied does not match what the boiler is set up to use, meaning it cannot operate safely.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

What the manual says

Manufacturer manual

Using the wrong gas

From this model's manual
Verified against the Navien Ncb 28ldwe installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

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

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