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The line carrying fuel vapour from the charcoal canister to the intake, restricted so purging cannot happen at the commanded rate. The purge valve opens correctly and nothing flows, so the module reports insufficient purge flow with a healthy valve fitted.

A canister that cannot be purged saturates, and a saturated canister stops absorbing vapour and starts passing raw fuel — so a restriction here eventually produces rich running and vapour escaping to atmosphere. The fault therefore migrates from a monitor failure to a driveability complaint if it is left, which is worth knowing when deciding how urgent it is.

The restriction is usually mechanical and outside the engine bay. These lines run under the vehicle to the canister near the tank, where they are crushed by a jack, kinked during other work, or blocked by debris or insects at an open end. Blow through the line with it disconnected at both ends — that settles it immediately, and check the canister's own inlet at the same time, since a saturated canister restricts as effectively as a crushed pipe.