Alec of the Technology Connections YouTube channel got a check engine light and the dreaded P0420 code on his Nissan Cube. He’s best known for his deep dives into how household appliances and hopelessly outdated electronics work, though he has occasionally ranted about automotive subjects like brake lights and turn signals. Now, he’s turned his attention to the catalytic converter, that expensive thing under your car that gets stolen all the time and helps with emissions, to figure out why his Cube has this code. After watching this, I understand how catalytic converters work better than ever.
He starts by explaining what’s inside a catalytic converter. We’ve covered this before;, but as a quick refresher it’s a ceramic honeycomb structure coated with platinum, rhodium, and palladium, precious metals that are the reason cats get stolen so often. These metals act as a catalyst that, particularly when combined with high exhaust temperatures, convert unburnt fuel and harmful nitrogen oxides that produce smog and acid rain into nitrogen, carbon dioxide, and water vapor, which is why your tailpipe sometimes drips water. This chemical reaction takes place automatically, but the engine computer has to do a lot of work behind the scenes to make it work effectively, efficiently, and last more than 100,000 miles in most cases.
It’s all computer
Thanks to modern fuel injection, the engine computer has extremely precise control over the proportions of the air/fuel mixture inside the cylinders. A countless number of sensors around the car tell the computer how well it’s doing, but the ones we’re concerned with here are the oxygen sensors before and after the catalytic converter (or converters, as some engines, particularly V6s and V8s, have two, one on each side). By comparing how much oxygen goes into the cat and how much comes out, which should be close to none, the computer can adjust the mixture to optimize efficiency in the cat, as well as the engine itself. When the computer’s best efforts can’t dial the oxygen out of the back side of the cat, it triggers the P0420 code.
There’s a whole lot more in this video, including how to tell if the problem is a bad sensor, an exhaust leak, or that the catalytic converter itself has failed. While Alec uses a professional-grade OBD2 scanner to diagnose that he did, in fact, have a bad cat, a consumer scanner will let you do the same thing. After watching this, I want my wife to take me for a drive in her 4Runner so I can monitor the oxygen sensor data and diagnose her P0420 code. I hope her cat isn’t cooked, but it might be, because it failed after some misfires occurred. I replaced all the old coils to fix that problem, but unburnt fuel during the misfires might have fouled the cat, which is something else I learned from this video.
