سوخت با اکتان پایین یا بی‌کیفیت

Fuel of a lower octane rating than the engine needs, or fuel that has degraded. Octane is a measure of resistance to detonation, so fuel below the specified rating ignites early under compression — and the knock sensors hear exactly that.

The module's response is to retard timing, which protects the engine and costs power and economy, so the complaint is a car that feels flat and drinks more, with knock codes and no mechanical fault. A fault that began immediately after a fill-up is worth taking at face value.

Degraded fuel reaches the same place by another route. Petrol left standing for months loses its light fractions and its effective octane, so a vehicle recommissioned after a long lay-up knocks on fuel that was fine when it went in. Drain and refill with fresh fuel of the correct grade before pursuing anything else — it is cheap, it is quick, and it either fixes the fault or eliminates it definitively.