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Q: Knocking noise when driving

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I had two oxygen sensors replaced, the engine decarbed, and then got a smog check. When I got it back, I could hear a knocking noise. It sounds like a rod rapping when accelerating in neutral. It will also make the noise while driving.

A: Hello. If you have a knocking noise coming ...

Hello. If you have a knocking noise coming from the engine then there may be a pending engine failure. If this occurred right after the sensor were replaced and after the smog test then there may be something else going on. If there is now a fuel mixture issue from the wrong sensors being installed then that can cause a misfire and a noise like this. I would typically install a computer scan tool first to see if there are any codes and see what the fuel mixture is. In the meantime you need to check the engine oil level to make sure that it is not low on oil. I would recommend having a mechanic, such as one from YourMechanic, come and inspect the knocking noise to determine what should be fixed.

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