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We recently had the knock sensor unit replaced. We had been driving the car for an hour. Suddenly the revs increased but the speed of the car wasn't increasing. the revs increased to 5000rpm and was only going 40kmph. Any ideas to what it might be?

Hi there, it sounds like a transmission type malfunction, or it could also be low on fluid or have a blockage.
It would be best to have the system checked out as it could be in a fail safe mode.