Odd DME behavior-emission issues
 

I've had my 01 Boxster S for about 5 years now and it recently turned 80K miles. About a year after I bought it the CEL came on and threw code P0420 several times, so the shop recommended I drive the car harder whicht seemed to clear the problem. Then it threw the same code and we replaced both pre-cat O2 sensors at 70 K miles.

About 6 mos. later the same code showed up again. We cleared the code and months later the CEL came on again and said the Bank 2 Cat was aging. I put some fuel additive in it which seemed to have helped. When I went to smog the car for CA registration about 18 mos ago, it wouldn't pass. I had the MAF and air filter cleaned; it turned out the alternator was on the way out and wasn't charging properly which is why it wouldn't pass smog. Once that was corrected it passed smog with nearly new car emission stats!


A year or so later it threw the P0420 code again and it was decided to swap the O2 sensors around. The car has an oil service with fuel additive at that point as well and several months went by without a problem.


And then once again the aging O2 sensor code showed up but on Bank 2. We replaced it with a previously used one which seemed to help until a week or so ago when it threw the same code. The shop decided to reinstall and update the DME software and a few days later the CEL came on again. Today when I went to the shop it showed code P0830 which states BOTH cats are bad. DOH!!

In all my years driving I've never had a car with such a weird problem. The car runs great, handles like a gem but this is crazy! I'm NOT replacing the CATS on this car that's for sure! Any ideas gang?

I'm afraid you are somewhat chasing your tail here. P0420 (and P0430) are codes for aging catalytic converters. Long before any sensors were changed, your tech needed to run a scan on the voltages of the before and after cat sensors; if the sensors are running at the correct voltages, there is nothing wrong with them. In my experience, gas additives and driving the car hard are band aids that seldom work. Do a search on these codes, this comes up from time to time, usually with the same end result.

http://i1140.photobucket.com/albums/...ps6728ac22.jpg http://i1140.photobucket.com/albums/...pscd3fb45b.jpg

After speaking with the chief technician, very in depth voltage scans of all sensors etc were done and checked out.

Furthermore I incorrectly stated the codes that have been showing up most of the time which are: P0133 (oxygen sensor ageing bank 1); P1275 (oxygen sensor ageing delay, bank 1) It threw these two codes in the late Spring. Shortly afterward the car was serviced and the bank 1 sensor was swapped to the other side. A few months later it threw the code again and it had moved along with the sensor. Then they replaced the sensor with a used one and it seemed OK until a week or so ago, when it threw the ageing code again. At that point they updated the DME and this brings us to the current situation.

The technician that works on the car is VERY seasoned, honest and wants it to work as it should. He has said he hasn't seen this sort of odd behavior before though the gas mixtures in CA change from season to season and the car does not get driven very often either. Hope that fills in the missing blanks.

Why would you replace a bad sensor with a used sensor? Perhaps you just answered your own question?

Probably need to find you a good price on replacement cats .$orry.
"Good used" may be an oxymoron.
There are hack cat-cleaning methods involving laundry detergent(!) but probably not advisable?

They replaced the sensor with a used one as a test to see if the code was thrown again with a different sensor. Since it was, it seemed logical that the problem doesn't seem to be sensor related but instead wiring or DME issues. This is why the next step was to update the software and in doing so, it throws a completely different code than ever before. FYI, the sensor ageing code was showing up regularly while the Cat Ageing only happened once a couple of years or more ago.

I've got the car on the market right now but in the meantime I'm keeping my fingers crossed. Even the very savvy technician says it's a rather odd problem since it's been inconsistent.