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Fault Code Analysis


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As long as I am getting such great advice about other topics I thought I would run this by for comment

Background - Car: 2001 Boxster S. 6-speed manual, 25,500 miles. Purchased in 2005 with 8300 miles on it. slightly modified by previous owner who was the owner of our local Porsche performance shop. Modifications include aftermarket K&N cold air intake, performance chip, aftermarket exhaust and suspension.

No problem for the first 2 years or so. First indicator light I took it in to local Porshe dealer and they said it was a bad O2 sensor. I was a newbie and did not ask what the fault code was. They changed it ad charged me $600. Everything was good for about 6 months. Second CEL I got wise and got my own Code reader. It was a Proscan unit that indicated fault codes of 1128 and 1130. Based on information I got from PCA tech support I was pretty sure it was either a leaky oil separator bellows hose or the MAF sensor. I cleaned the MAF sensor put it back in and reset the codes. About two weeks later (about the time it takes for me to go through the driving cycles based on my driving pattern) the CEL light came on again. I figured the MAF was faulty and not just dirty so I ordered another one. I was told that the MAF unit in my car had been superseded by another part number (986-606-125-01).

I had read that if the newer part were installed that the DME would need to be reprogrammed. I contacted PCA tech support again and asked if this was true, and they indicated that only cars built before January 2000 would need to have the DME reprogrammed so I felt I was good to go.

Installed the new unit and in about 2 weeks, the CEL came on again. This time I had an indication of aging post cat O2 sensors. Both Banks. With only 25k miles on it I was doubtful that this was the problem, bu rationalized that it could be possible. I bit the bullet and replaced the post cat O2 sensors and two weeks later I got a new set of codes. This time I was using a Durametric Analyzer software version 5.1.4.0. The codes recorded were P0133 - Aging Oxygen Sensor Ahead of Cat. (Cylinders 1-3) below lower limit, P0420 - exceeds limit value, and P0430 - exceeds limit value.

The car seems to be running fine and as a result, I have not done much to chase the problem, but I'm getting close to re-registering the car and need to track it down in order to pass smog. I was contemplating having the oil separator bellows hose replaced just as a matter of principal at my next oil change, but other than that, I am at a loss for what to do next. I am willing to try anything that is relatively easy that I may have overlooked, but otherwise I will need to take it in an have it thoroughly gone through to eradicate the problem once and for all.

Any help or advice you might give would be appreciated.

Thank you.

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P0420 and P0430 are cat efficiency codes which usually indicate failing cats. With all of the problems you've been having with the your o2 sensors and cats, I would wonder if the aftermarket equipment isn't causing issues? That being said I would check your exhaust system over for leaks. A small exhaust leak will play with the sensor readings but even then they usually throw mixture related faults. You could try swapping the pre cat sensors between bank 1 and bank 2 and see if your P0133 switches over to bank 2. That should confirm if you really just a bad sensor or some other fault. As for the cat efficiency faults, usually by the time they throw these faults the material inside the cat is usually loose or visibly damaged. You could try banging on the cat with a rubber mallet and listening for anything loose sounding. If it sounds loose - you need new cat$.

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  • 2 weeks later...

It is rare for CATS to go bad unless your car ran so rich that it dumped raw fuel into them over a period of time. I'm not sure about your reprogramming advice reg. the MAF because my understanding is just the opposite of what you got. Reprogramming has to be done on 2000 cars and up because they went to an electronic fuel delivery system and that is why the MAF designation changed. 97-99 year cars still used a fly-by-wire system throttle and don't need to be reprogrammed as long as the original part number MAF is installed. Emission control codes are a ***** to track down and typically aren't found until a smoke test and vacuum leak test is done on the motor and exhaust systems. I had similar codes to yours and it took two smoke tests and a vacuum leak test to determine it was the Air/Oil separator bellows and a broken vacuum line from a valve in the secondary air system. It basically took almost $400 to find and replace about $20 in parts but I did resist several recommendations to replace cats and O2 sensors, at a cost of around $3K, until all other tests were exhausted. The problem with a lot of dealers (and unfortunately many independent mechanics I'm finding) is they are trained to replace systems instead of faulty parts as a system replacement means more profit and it is easier to diagnose a problem and fix it if everything is replaced. Find a good independent mechanic who will diagnose a system to find specific problems and replace or repair them. In the long run it will save you money, headaches and downtime.

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