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A hose in the evaporative system off its stub, leaving the sealed system open to atmosphere. The system's whole function is to be sealed, so a disconnected hose is the largest leak it can report and produces a gross leak code immediately.

Where it disconnects decides the second symptom. A purge hose off admits unmetered air to the intake, so the idle hunts and the mixture goes lean; a hose on the tank side merely vents vapour and affects nothing the driver notices. A large evaporative leak together with a lean idle locates it on the purge side without any further testing.

Recent work is the strongest predictor, since these hoses are removed to reach the intake, the canister or the tank and are easy to leave unclipped. Ageing is the other route: the rubber hardens and the hose releases from its stub. Check the whole run rather than the ends — and a smoke test finds a disconnected hose instantly, which is worth doing before searching by hand under the vehicle.