P0301 Error Code in 2000 Chrysler Intrepid: Diagnosis and Repair Guide

Hello everyone, I'm new to this forum, so please forgive any mistakes. I recently purchased a 2000 Chrysler Intrepid with a 3.2L engine, originally driven at 165,000km. The car was rebuilt by a mechanic and his son after the original engine failed — they installed a used replacement engine from the same year and make and performed extensive repairs. Over the past three weeks, I've been experiencing a persistent P0301 diagnostic trouble code (DTC). Prior to that, I noticed intermittent hesitation and sputtering during high-speed driving—especially when passing other vehicles. The check engine light would blink intermittently but would turn off once I stabilized my speed. Now, the check engine light is permanently on, and the vehicle shakes and sputters under all conditions. This weekend marks my planned 'Operation' to address it. I'm looking for practical troubleshooting steps to diagnose this issue myself. While I've done mechanical work before—such as rebuilding a starter and alternator, replacing wires, distributor, radiator, hoses, belts, water pump, thermostat, fuel pump, and fuel filter on my 1985 Pontiac Parisienne—the Intrepid is quite different, especially with its fuel-injected system. I’ve reviewed various sources and found nine possible causes for P0301: spark plugs, ignition wires, fuel injectors, or even a faulty engine control module. How can I isolate the root cause more efficiently? Any advice on where to start—especially for someone with limited experience in modern fuel-injected vehicles? Thanks in advance! P.S.: I'm curious about oxygen sensor lifespan and whether it could be contributing to these symptoms. My research shows there are four O2 sensors: two before the catalytic converter and two after. I also plan to replace the exhaust system from the converter onward—so I’d like to include that in my cost estimates. Thanks again for any insights!