P0300 and P0420 Fault Diagnosis for 2004 Pontiac GTP Wide Track with 170,000 Miles
I recently purchased a 2004 Pontiac GTP Wide Track with 170,000 miles, featuring a supercharged 3.8L engine. The car was in excellent condition overall—only visible wear is on the steering wheel, brake pedal, and radio buttons. After bringing it back from the shop a week ago, I noticed the vehicle was running poorly and struggling to accelerate. The mechanic replaced the spark plugs, ignition wires, and one fuel injector. However, the performance issue persists, and the check engine light is now on. Using an OBD2 scanner, I detected two fault codes: P0300 (random misfire) and P0420 (catalyst efficiency below threshold for Bank 1). I decided not to return to the shop because they held the vehicle for three weeks and charged $400. I'm seeking advice on how to properly diagnose and fix these issues, especially since a misfire could be damaging the catalytic converter. Any insights or troubleshooting steps would be greatly appreciated!
The P0300 code indicates a random cylinder misfire, which is likely causing damage to the catalytic converter—hence the P0420 code. To confirm this, remove the upstream oxygen sensor on Bank 1 and test if the vehicle runs smoother. If performance improves without the sensor, it strongly suggests that the misfire is the root cause. In that case, you'll need a new catalytic converter. However, be sure to address and fix the underlying misfire issue (such as faulty spark plugs or ignition components) before installing a replacement—otherwise, the new converter will likely fail quickly.