Most likely would be sensor IMO, although that is probably the most difficult / expensive to change so worth checking others first.
If you have a spare ECU, it's worth a try just in case - especially if this one hasn't been used for a while.
Another prime candidate would be a short to ground at the knock sensor end - especially likely as many people don't realise it's screened cable and re-make the end with screen and core all connected together.
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