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Fuel that has degraded in storage or carries contamination. Petrol loses its light fractions over months, so a vehicle recommissioned after a lay-up runs on fuel with a lower effective octane and poor volatility — which produces knock and hard starting on fuel that was fine when it went in.

The signature is a fault that begins on the first run after standing, or immediately after a fill from a low-turnover pump. Knock codes across several cylinders with no mechanical fault point at the fuel before they point at sensors. Drain and refill with fresh fuel of the correct grade — it is cheap and quick and either fixes the fault or eliminates it definitively, which is worth doing before a knock sensor is condemned.