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The vaporizer — installers usually call it the reducer — that turns liquid LPG into gas has failed, scaled up internally, or lost its heating.

It is the part of a gas system with no petrol equivalent, and understanding it explains most of how a bi-fuel car behaves. LPG leaves the tank as a liquid and must reach the injectors as a gas. Boiling it off absorbs heat — the same physics that frosts a gas cylinder when you draw from it hard — so left alone a vaporizer would freeze itself solid within a minute.

That is why it is plumbed into the engine coolant circuit, and why a bi-fuel car always starts on petrol: until the coolant is warm there is no gas to burn. It also means a vaporizer fault and a cooling-system fault are hard to tell apart from the symptoms. A thermostat stuck open delays changeover on every journey while producing no other complaint anyone connects to it — worth eliminating before the vaporizer is condemned.

Its own failures are two. It scales up internally over years, transferring heat less well, which stretches changeover time so gradually that nobody can date it. Or its diaphragm perishes, which is the one that matters: a failed diaphragm can pass gas into the coolant circuit, so a converted car whose cooling system mysteriously pressurises is worth taking seriously.

These are usually aftermarket installations, so the work belongs with a gas installer rather than a general workshop.