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2000 Boxster Oxygen Sensor

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My 2000 (2.7L) Boxster has CE light and my OBD2 tool gives P1128.

Acording to my bentley Service Manual I have DME ME 7.2 DTCs and the code reads "Oxygen Sensing Area 2, Cylinders 1-3".

I have 2 new Bosch Oxygen Sensors (up/downstream) and I was preparing to replace both sensors - but suddenly I am second guessing myself. Here are the questions I would appreciate input on.

1) The CE light came on just after I stalled the car at a stop light. Talking on the phone is my very poor excuse. I am wondering if this is related. Should I reset the CE light and see if it comes back on? Could the stall have caused a false alarm?

2) Where is Area 2. I assume this is downstream T/F?

3) Should I replace both sensors for bank 1 or just the affected one? My car has just 30K miles and I think this is too soon for O2 sensors to fail.

4) Should I in fact think about changing all 4 sensors?

5) Which side of the car are cylinders 1-3 (bank 1) located?

6) Is there something else I should look at? Is it conceivable that I have a blocked air filter or some other issue that would save me a trip under the car.

I might add that I bought the car just weeks ago. I am secretly happy that there is an issue, because I want to "tinker" with it as much as I want to drive it. That said, I am not that keen on replacing parts that may actually be okay.

Any guidance and comments (other than clutch control at traffic lights) would be appreciated.

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P1128 Oxygen Sensing Adaptation, Idle Range, Bank 1 – Above Limit

What does that mean:

It does NOT mean you have a bad O2 sensor. The O2 sensors report the fault - they are not the fault in this case.

The fuel/air mixture is so lean that sensing has reached its rich threshold. That is, the DME thinks the mixture is too lean so it has richened the mixture to the point it can not make it any richer. Chances are the MAF is giving a faulty signal to the DME.

Potential causes (from the Porsche DME 7.2 OBD II manual):

– Incorrect signal from MAF sensor

– Intake air system leaking

– Fuel pressure too low

– Volume supply of fuel pump too low

– Fuel injectors fouled

– Exhaust system leaking

If that is the only fault you have I would start by cleaning the MAF or replacing it.

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I suspect that you are a wiser man than than I, but are you sure about the code meaning:

P1128 Oxygen Sensing Adaptation, Idle Range, Bank 1 – Above Limit

I have no idea how reliable the Bentley Manual is, but it quotes this code exactly for DME M 5.2.2, but for 2000-2002 models it has the interpretation I quoted.

I would of course believe the Porsche DME 7.2 OBD II manual but before thought I would double check your resource with you.

Cleaning the MAF looks like a simple project. Replacing it could be expensive.

What do you think of my idea of resetting the CEL and seeing what happens. Is this a good strategy?

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If your car is a MY2000 Boxster then it has e-gas and then it must be a DME 7.2.

That section of the Porsche DME 7.2 OBD II manual was updated in supplement 3 sometime in 1999. My guess is that Bentley got the information from an older manual.

I would clean the MAF (do a search here as this has been covered many times) -- then reset the CEL.

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Mike, Loren,

Thanks for the help. I removed the MAF and cleaned it per Mike's excellent instructions (the wire/plug) did not come apart from the MAF quite as described, but no big deal.

It was reassuring to see that the engine is really clean (at 29k miles) as was the MAF, airfilter etc.

I have reset the CEL and done a couple of test drives. So far no CEL, but I guess I need a few miles under me before I will know.

One thing that did occur to me AFTER the fact. The last couple of times I started the car before the problem occured, I could smell gas. Since the cleaning there has been no smell. I take that as a good sign.

I ned to let you guys know about the great service from Smith Motorworks in FL. A GREAT place to buy parts from. I will cover this in another post.

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