Cause
Low octane or poor quality fuel
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.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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