Cause
Stale or contaminated fuel
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.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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