Originally posted by Big Jim:
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.
Interesting. Thanks Jim. I'm just wondering if one of O2 sensors wasn't accurate - not completely bad that it would set an O2 sensor code, but bad in that it was giving inaccurate readings, that the computer would interpret that as a bad cat.
I just see no reason why the rear precat would fail after only 58,00 miles on an engine that runs perfect and doesnt burn any oil. Unless the dealer replaced my previous bad cat with another defective cat? But this ones definitely not plugged up - the defective ones were.
I'm tempted to just buy a new O2 sensor - not that those little buggers are cheap, but cheaper than a new precat.
