1999 GMC Yukon P0420 Code: Is It a Faulty Oxygen Sensor or a Bad Catalytic Converter?
I recently purchased a 1999 GMC Yukon from a friend who previously attempted to replace the spark plugs. During that process, he incorrectly installed the spark plug wires, resulting in a firing order fault and a blinking check engine light (MIL). After correcting the wiring issue, I noticed a P0131 code (downstream oxygen sensor on driver's side) last week. The vehicle had been idle for several months with approximately 150,000 miles, so I replaced the downstream oxygen sensor behind the catalytic converter—this cleared the P0131 code. However, a new P0420 code has now appeared. I'm concerned whether this could be due to the quality of the original oxygen sensor, which I purchased from AutoZone (Bosch brand). I've since ordered ACDelco sensors for replacement and plan to return the Bosch units once they arrive. My main question is: Is it more likely that the P0420 code stems from a faulty new sensor, or is it actually indicating a failing catalytic converter? The original P0131 may have been caused by a deteriorated converter, which would explain why the downstream O2 sensor was showing issues. I have some mechanical experience—having performed a motor swap on a Grand Prix before. If the issue turns out to be a bad catalytic converter, is it something a DIYer like me could handle? I initially thought catalytic converters required welding, but online replacement guides suggest they are bolt-in units. I'll confirm this when I replace the O2 sensor again.
Given the vehicle's age and history of a catalyst-damaging misfire, it's not surprising that the catalytic converter might be failing. Bosch oxygen sensors are widely considered low quality—especially at auto parts stores like AutoZone. If you're considering replacing them first, do so, clear the codes, and monitor whether the P0420 returns. You may get lucky and see no recurrence, but it's likely that the catalytic converter is on its last legs.