I have changed my fuel pump, fuel filter, cam and crankshaft sensors, put new spark plugs, engine coolant sensor, and o2 sensors. My check engine is still on for the o2 sensors I changed. I also got a engine misfire code and fuel rail code. I took it to a mechanic and he told me that 3 of my pistons were bad but I check my engine compression and it was fine. Can it be the pistons? Or what else can cause this?