Q: Car lacks power

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My car has been losing power. It started with just a stutter once in a while. It has turned into something much more serious. It has thrown the codes P0301, P0302, and P0420. I have replaced the spark plugs recently and the engine stopped acting up for about 50 miles. The ignition system was replaced 40,000 miles ago, the O2 sensor has been replaced as well. I thought it was the catalytic converter, but now I'm wondering if it's the fuel pump. Once the engine warms up to normal operating temperatures it runs smooth. I'm wondering if the fuel pump is causing the misfires which is causing the P0420 code to pop up. If it is the fuel pump why would it stop acting up once the engine warms up. It also tend to die whenever I have to stop.

My car has 164600 miles.
My car has a manual transmission.

First check your EGR system for leaks. If EGR is on or stuck, at idle, that can cause rough operation. To further diagnose your issue, obtain compression test results (in case the issue is due, even in part, to burned exhaust valve(s)) and put the engine on a scope to evaluate the cylinder firing patterns, which scope exam would distinguish between fuel and/or ignition system faults. All the foregoing can be accomplished if you request an engine stalling diagnostic. Your mention of a difference in cold versus hot (normal) operation might also occur if there is a bad sensor misreporting the engine coolant temperature to the PCM or a sensor that is malfunctioning once heated (resistance, including at harness terminals, increases with heat). Taken together, the Codes you report are also consistent with a possible clogged fuel filter, bad fuel pump, and vacuum leaks, including, as noted, via a stuck open EGR system. The bottom line is you should request an engine stalling diagnostic very simply because all of these possibilities have to be looked at sequentially on a "decision tree", that is in a methodical fashion. Please let us know if you have further concerns or follow up questions.

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