A few months ago, my check engine light came on after a long freeway drive and about four miles off the freeway. I had it scanned and got the same results reported here. I also scanned the operation of the oxygen sensors and found that the rear sensor was switching frequently showing that the cat was not doing well. I cleared the code and drove the car for several weeks before the light came on again. I read the codes again and scanned the operation of the oxygen sensors and found the same problem, which is what I expected. The cat monitor system does not constantly check the cat operation. There is a certain sequence of driving that must take place before the cat monitor will look at those readings. So I expected that I would probably need to buy a rear pre cat before the next time the car needed to be smoged. I cleared the code again.
About two weeks ago the light came on again and I read the codes and found that the light was set by a loose gas cap. My daughter had filled the tank last and didn't get the cap on straight. Since I had the scanner hooked up, I took a look at the oxygen sensors and to my surprise I found them to be reading properly, indicating that the rear cat was functining. With a little bit of luck, I can put off the replacement of the rear pre cat for a while longer. I have no idea why it is working more effeciently now than it was a few weeks ago.
