It's not an English forum. I'm just wondering why so many people are using the same wrong terminology over and over and finally decided to ask. Since 1996 models, all cars sold in the U.S. Have the OBD2 (on-board diagnostics version 2), emissions system which includes a sensor like normal in front of the catalytic converter to fine tune the fuel / air mixture. The second one added after the converter is to monitor the catalyst efficiency. The front one will switch between "rich" and "lean" multiple times per second. When the converter is working properly the exhaust gas leaving it will switch very slowly, perhaps as little as once or twice every few minutes. When the converter stops working no change takes place in the composition of the exhaust gas so the second, (downstream) sensor switches faster. When it approaches the switching rate of the front sensor the computer knows no change is taking place in the gases, it sets a related diagnostic fault code in memory, and turns on the Check Engine light.
Tuesday, March 15th, 2011 AT 11:22 PM