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Q: What is causing the engine misfires?

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I had a P0301 code so I changed all spark plugs and the coil for cylinder 1. Then, I cleared the code. Soon after a P0303 code popped up. I've been having low RPM at stops, poor gas mileage and a lot of smoke out of the muffler when I start the vehicle in the morning. What could be the cause? Camshaft sensor? Leak?

My car has 143560 miles.
My car has an automatic transmission.

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