Usually if your check engine light is on that means you need some repair work done on your car but that is not always the case. I drive a 2002 Chevy Impala and the check engine light has been coming on for about 4 years. The trouble code is "P0420". That means the catalytic converter is functioning below the normal efficiency levels. Back in 2007 when I first noticed the problem I had the catalytic converter replaced. At that time the performance of the Impala was greatly reduced. Specifically when ever the accelerator was pressed there was a significant time delay before the car would accelerate. It almost seemed as if the transmission was slipping and then suddenly it would catch and the car would accelerate. The problem however was not the automatic transmission. The problem was the catalytic converter. There was a carbon blockage that clogged the air flowing through the power train. Exhaust was escaping through the EGR valve because the catalytic convert was blocked up so much. So I bought the new catalytic converter. The cost was roughly $800 as I learned that my vehicle was manufactured to the California emissions standards and not the federal emissions standars. That was news to me.
About two or three months after the catalytic convert was replaced the check engine light came on again. This is around the time I asked for an OBD-II code scanner for Christmas. The same "P0240" trouble code was being stored in the engine control module. The catalytic converter was under warranty so I had it replaced thinking it may have been defective. Again the "P0420" trouble code came back. At this point I started to see a pattern.
There was no performance problem with my car anymore. That had been resolved after the first catalytic converter was installed. The only issue was that the check engine light kept coming on. The Oxygen Sensors before and after the catalytic convert have been tested and I was assured they were functioning properly. If the Oxygen Sensors are out of whack that could possibly trick the engine control module into thinking that there is a problem with the catalytic converter. So that was ruled out.
To this day in order to pass inspection I have to erase the check engine light and complete a drive cycle. Only then will my car pass inspection. Of course two weeks after the inspection the check engine light will come back on. Hope this informations helps someone out, or maybe you have your own theory about whats really wrong. I would love to hear your thoughts.
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