Cause
Restricted alternative fuel return line
The return line in a gaseous fuel system restricted, so unused fuel cannot flow back. It is the line rather than the valve, which is the distinction — the valve is commanded and monitored, and the line is passive and therefore invisible to every electrical test.
Restricted, pressure builds where it should be relieved, and the module reports insufficient flow with a valve that operates correctly. These systems hold fuel at pressures far above a petrol system, so depressurise by the correct procedure before opening anything. The restriction is usually mechanical: a crushed or kinked line under the vehicle, or corrosion at a fitting. Most installations are aftermarket, so trace the actual pipework rather than a factory diagram and treat the installer's documentation as the reference.
Typically specialist work. Needs equipment or training most home workshops do not have.
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