There's two ways they control tail pipe emissions. We're all familiar with computer engine controls that regulate fuel metering for cleanest combustion, but there is still always going to be some pollutants that must be cleaned up afterwards. That's the job of the catalytic converter. Those have been around since the 1970s, but it was with all '96 models that we added a system to monitor how well those converters are working. The engine can run just fine, as you've observed, but the Engine Computer is not happy with what it sees related to that converter.
The way this is done is once the engine is up to temperature, the Engine Computer commands a fuel / air mixture that's slightly too lean, then slightly too rich, then it expects to see that reported by the first, or "upstream" oxygen sensor. That switching occurs roughly twice per second and can be observed on a scanner. When the exhaust gas goes too lean, the extra, unburned oxygen is stored in the catalyst material. When the exhaust goes too rich, the unburned fuel mixes with that stored oxygen and is burned. Ideally that results in an exhaust pipe so clean, you could suck on it and live to tell about it.
What was added on '96 models was the addition of the second, or "downstream" oxygen sensor. That one could be the same part number as the upstream sensor, but it has nothing to do with engine performance. When the converter is working properly, that downstream sensor might gradually switch from a little rich to a little lean perhaps once every minute or two. The switch rate is very long compared to the front sensor that switches multiple times per second.
When something happens to the catalyst and it no longer cleans up the exhaust gas, what comes out is the same as what went in. When no change takes place, both sensors switch at the same rate. That doesn't happen instantly. Rather, the downstream sensor switches between "rich" and "lean" a little faster, and a little faster, as the converter's efficiency drops. There is a point in the software where the Engine Computer figures out too much pollutants are leaving the tail pipe, or, in this case, it doesn't know if the exhaust gas is being cleaned up. If there are no other fault codes to point to a cause of the excessive emissions, or the lack of information, the only thing left is usually the converter.
There are a number of aftermarket suppliers for new catalytic converters, but I've read quite a few stories of them not working properly on import vehicles. This may have to do with every vehicle has a converter designed and tailored for exactly that engine, and that's what you get when you buy a new one from the dealer. For the aftermarket suppliers, rather than try to build thousands of different converters, and never have the right one in stock, they try to design a few models to cover a wide range of applications. One might work on one car model just fine, while on another car, it can clean the exhaust but not in a way the Engine Computer wants to see it. It's also not uncommon to have an aftermarket converter work okay for a few months, then the fault codes start setting again. One clue to what you're getting is a converter designed specifically for your car and engine size is usually a simple bolt-on affair. With a design meant to fit many models, the front and rear pipes might have to be welded on first, and you likely will have to buy those pipes according to the application.
If you have a trusted mechanic, go by his recommendation as to the best converter. He will want to perform a few tests first to verify the new converter is needed.
Please keep me updated on how this turns out.
Thursday, January 8th, 2026 AT 8:28 PM
