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1998 986 Failed Emissions Test, P1123, P1125, P0150

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I've got a '98 Boxster with the manual 5 speed transmission. It recently failed an emissions test due to CEL being on and fault code P1123.


Based on the advise in another post in this forum, I removed and cleaned the MAF sensor. I used my Durametric cable to clear the codes and took it for a 30 minute test drive. After that, I used the Durametric Short Test and am still getting P1123 as well as P1125 and P0150.


After more searching, I found posts that suggested a vacuum leak. I don't have a smoke tester, but with the car idling, I unscrewed the oil filler cap and the engine changed RPM's. I also tested by loosening a host that goes to the air-oil separator and the RPM's changed again. My understanding is that indicates a vacuum leak is less likely.


More searching and I found Mike Focke's post here:



Using the Durametric software, I took the following measurements:

02 sensor voltages ahead of cat. c. bank 2
oscillates between approx 0.1 V and 0.8 V
02 sensor voltages ahead of cat. conv.
oscillates between approx 0.05 and 0.75
period seems a little shorter than the other sensor
Oxygen sensing cylinders 4-6
steady between about 1.0V and 1.06V
Oxygen sensing cylinders 1-3
steady between about 0.95V and 1.0V
Based on my understanding of Focke's post, the O2 sensors ahead of the cats are reading okay (although I don't know about the frequency of oscillation... could they be "tired"?. Also based on Focke's post, the last two Oxygen sensor measures look suspicious. They are supposed to be reading a voltage close to 0V, but are reading voltages close to 1V. In Focke's post, these sensors may oscillate if the cat is not working correctly. So, what does a steady high reading indicate? Maybe I'm not looking at the correct measures in the Durametric software?
Any idea what's wrong?
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You have codes of overly rich conditions on  both banks (1123 and 1125), and a code for the O2 sensor ahead of the main cat on the bank for cylinders 4-6 (P0150) either being short circuited or having lower than expected voltage.  I would start with the P0150 code and check the harness and the sensor itself for a short, and the short could be the sensor itself.

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  • 2 weeks later...

Couple of things to consider

1. Oxygen sensors are not expensive and easy to replace - do ll 4.

2. This makes subsequent diagnosis easier because it eliminates the O2 sensors - but not their wiring,

Check the plugs, tubes+sels and coil packs at the same time ?

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