1999 Oldsmobile Intrigue 3800 V6 with P0420 After Ignition Module Failure and Fuel Issues
I own a 1999 Oldsmobile Intrigue with a 3800 V6 engine, automatic transmission, and approximately 114,000 miles. Historically, this vehicle has been very reliable—never experiencing oil leaks, coolant loss, or major mechanical issues. It had a full 100,000-mile tune-up, but recently began exhibiting serious performance problems. About two weeks ago, during a 250-mile trip, the car started missing and stumbling severely right after refueling at my destination. It was nearly impossible to move beyond the filling station—only able to idle roughly. At that time, no Check Engine light was on. My initial suspicion was water contamination in the new fuel, so I added two bottles of STP Heet. However, this did not resolve the issue. I then drove to a nearby GM dealership and asked my parents for assistance—they temporarily swapped me with one of their spare vehicles while I waited. The dealer diagnosed a failed ignition module, which they replaced—this part had not been serviced during the previous tune-up. They noted it was an unexpected failure that occurred in a parking lot at the filling station. A week later, I picked up the car and drove 225 miles back home. After about 200 miles, the Check Engine light came on. I initially suspected this was a residual code from the ignition issue, so I disconnected the battery to reset it and ordered an OBD-II scanner. I also refilled the fuel tank with fresh gas, minimizing any remaining Heet. After another week, the Check Engine light returned—this time showing fault code P042 or P0420 (Catalyst System Efficiency Below Threshold). My questions are: 1. Is it highly unlikely that these two events—the ignition failure and the P0420—are unrelated? 2. Could the ignition module failure itself have contributed to catalytic converter damage? Or was the post-failure driving—especially the 1/2-mile attempt to coax the engine—possibly a contributing factor? Could STP Heet or fuel additives play a role? 3. I am still on my second tank of gas (less than 100 miles since the light first appeared). Will this code resolve itself temporarily, or should I consider replacing the catalytic converter? Thanks for any insight, Brian
It's likely that you had an underlying ignition system issue that may have damaged the catalytic converter. The failure of the ignition module could have caused misfires, which over time reduce catalyst efficiency and trigger a P0420 code. This event might simply have pushed the vehicle past its threshold for detecting performance degradation.