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The valve that isolates a fuel supply has stopped responding — stuck shut, stuck open, or electrically dead.

What it does depends on the engine, and the two cases have very different consequences.

On a diesel it is how the engine stops. A compression-ignition engine has no ignition to switch off, so cutting fuel is the only way to shut it down. A valve stuck closed gives a no-start; a valve stuck open can leave an engine that will not stop with the key, which is a genuine hazard rather than an inconvenience and a reason to have the vehicle recovered rather than driven.

On a bi-fuel vehicle it isolates a gaseous fuel at the tank, and there it is a safety device first. These valves are arranged to fail closed — no power means no fuel — so an electrical failure leaves the supply shut off rather than escaping, and the vehicle simply reverts to its other fuel and keeps running.

Before condemning one, note where it sits. A gas system's valve is mounted on a pressurised tank and its wiring runs under the floor of the vehicle through road salt, water and stone damage — so a wiring fault is far likelier than valve failure and vastly cheaper, while reaching the valve at all means depressurising the tank. That work needs an installer qualified on gas systems, not a general garage.