P0420 OBD2 Code on 2001 Oldsmobile Alero: Is My Catalytic Converter Really Faulty?

Amy Evans wrote: ... the catalytic converter is functioning correctly but ... How did you verify the converter's performance? P0420 typically indicates a failing catalytic converter. Have you reviewed this OBD2 code explanation yet? https://www.obd-codes.com/p0420

No, clearing the P0420 code won't fix it—this code is triggered when emissions monitoring shows the converter isn't performing adequately. While a working converter might pass tailpipe tests, the MIL stays on because the vehicle's onboard system detects failure in conversion efficiency.

The MIL remains on because the catalytic converter is likely not functioning properly. You can try clearing the code and driving under normal conditions to allow emission monitors to relearn, but if the issue persists, it will likely return.

asecmt wrote: ... the MIL stays on because the converter is failing. Clearing the code and driving may help temporarily—but I've replaced my catalytic converter three times now, and the light keeps coming back on! What should I do next?}

Many users replace converters repeatedly—often opting for universal-fit models from auto parts stores. But these often don't meet emissions standards and fail to resolve P0420 issues. Where did you source your converter(s) from?}

The downstream oxygen sensor (after the catalytic converter) is critical for monitoring conversion efficiency. The upstream sensor detects exhaust gas composition before the converter, and the downstream one checks after. If the downstream sensor shows fluctuating readings, it may be faulty or damaged during converter replacement. When you installed the new converter, did you replace just the converter tube or the full assembly? A spliced-in installation can damage nearby components. These sensors cost around $80—often available at salvage yards for less. They don’t typically fail unless physically compromised.

asecmt wrote: ... where did you get your converters? I bought three from my local parts store, which honored the warranty on the first two. First: $80 universal Maremont (spliced and welded). Second: $120 Magnaflow (universal, also spliced/welded). Third: a direct-fit Magnaflow that bolts on—welded at the splice after installation. The first two had splices, so I suspect the downstream oxygen sensor may have been damaged during installation. After three replacements, I'm starting to believe there's another underlying issue.

I suspect the second (downstream) oxygen sensor is providing incorrect readings—this could falsely trigger P0420 even if the converter itself is working properly.

If you have access to an OBD2 scanner, monitor the downstream oxygen sensor voltage. Under normal conditions, it should show a steady reading (close to 0.5V) after the catalytic converter. If it fluctuates significantly—similar to the upstream sensor—it indicates poor conversion efficiency and likely points to a failing converter or faulty sensor.

This might be a long shot—but on GM vehicles, especially older models like the Alero, lower intake manifold (LIM) gasket failures can cause coolant leaks. Coolant entering the engine bay and reaching the exhaust system (via intake valves) can degrade catalytic converter performance due to glycol exposure. Oil contamination can also damage converters over time.

Skydrol wrote: ... coolant leaks from LIM gaskets can destroy the catalytic converter. What engine do you have in your Alero? Magnaflow converters are often marketed for performance, not emissions compliance—using them may actually cause P0420 due to higher exhaust temperatures or poor flow characteristics.

I'm fairly confident this is a V6 engine based on common configurations in that model year and trim.

I recently ran into P0420 after replacing $600 worth of parts—including a Magnaflow converter welded onto the exhaust. I heard chunks of converter material rattling inside! I know upstream O2 should fluctuate and downstream should be steady—but if both sensors show similar readings, it will trigger the P0420 code. Driving a Saturn Vue (not recommended) with a 2.2L Ecotec engine (new & improved Quad4)—this is a real headache for owners.