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The valve controlling the return path in a gaseous fuel system — liquefied petroleum gas or compressed natural gas — which manages what happens to fuel not consumed. Alternative fuel systems are a genuinely separate circuit with their own tank, pressure regime and valves, so this is not the petrol return system under another name, and diagnosing it as one leads nowhere.

Its codes split the usual way. Circuit codes — open, low, high — are the winding and its harness, and this harness runs to a tank commonly mounted under the vehicle or in the boot, exposed to road salt and impact. An insufficient flow code is the valve body: it is electrically sound and not passing what it should, which is a stuck valve or a restriction in the line.

Two cautions specific to these systems. They hold fuel at pressures far above a petrol system, so the circuit must be depressurised by the correct procedure before anything is opened. And installations are frequently aftermarket, so components, wiring and routing may not match any factory documentation — trace the actual installation rather than a diagram, and establish who installed it, because the conversion's own paperwork is often the only accurate reference for the system.