Failed alternative fuel control module

The controller managing a bi-fuel installation — compressed natural gas, liquefied petroleum gas or similar — alongside the vehicle's petrol system.

The single most useful fact is that it is usually not the vehicle manufacturer's part. Most of these installations are conversions fitted by a specialist, with their own module, sensors, wiring and documentation. So the installer is the right first call, not a main dealer — who may have neither the diagrams nor the tooling, and where a fruitless diagnostic bill is a common outcome.

Before the module is suspected, two innocent explanations deserve elimination. A near-empty gas tank produces low pressure and level readings the system cannot reconcile, and filling it sometimes resolves everything. And extreme weather — gas tank pressure varies enormously with temperature, so a very hot or very cold spell can push a slightly drifted sensor outside acceptable limits while nothing has failed.

When the system does stop working, the vehicle reverts to petrol and keeps running, which is the designed behaviour: fuelling a gas system on figures it cannot trust risks running lean, and on a gaseous fuel that means high combustion temperatures and real engine damage.

Two reasons not to simply live on petrol. The running-cost saving that justified the installation is gone, and gas systems are subject to periodic inspection in most jurisdictions, which a stored fault will not pass. Work on the tank side needs someone qualified for it, since the valve is mounted on a pressurised vessel.