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3-Series (E36) Chat relating to the BMW 3-Series from 1992-1999. Autodoodad Specific models include: BMW 316i, BMW 318i, BMW 318iS/ti, BMW 320, BMW 323, BMW 320, BMW 324, BMW 325, BMW 328.

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Old 12-26-2004, 12:43 PM   #1 (permalink)
n00b
 
Join Date: Jun 2004
Location: Worthington, KY
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1998 BMW 323is / 5spd

I have a code check/light reset tool from Bavarian Auto, and I'm attempting to diagnose a code that continues to fault. The code is E9, and in my tool manual, states "Catalyst Efficiency Below Threshold, Cyl #1-3".

I thought this may have something to do with the Oxygen sensor as I have recently noticed my gas mileage drop a little (30 drop to 28). However, that may not exactly be as much due a bad sensor as it would be changing driving conditions. Bottom line, my car runs perfect, but this light keeps coming on, so there must be a problem somewhere.

Thanks in advance for the help.
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Old 12-26-2004, 07:38 PM   #2 (permalink)
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vmarks is an unknown quantity at this point
cat efficiency below threshold means one of the following:

-cat is bad, not being as effective as required at reducing emissions. most likely if your cat is really bad, you'd have a bad stink coming from the tail pipe (people say it's like rotten eggs).

-or, a post-cat o2 sensor is fouled, meaning that the computer is getting a reading that makes it think your cat efficiency is bad. you can find which sensor it is by tracing the exhaust manifold back from cylinders 1-3, and finding the post-cat sensor on that side.


in my owner's manual, it says o2 sensors should be changed every 90k miles. so if they haven't been changed yet and your miles are up there, go ahead and do it. you can use a box wrench; i changed all 4 of mine using a box wrench.

pre-cat and post-cat sensors are the same, except one of them has longer wires (i think the pre-cat are longer wires). the place i ordered mine from just sends the pre-cat sensors no matter which are ordered. i think most places now do that, they just stock the ones with the longest wires.

also, as far as o2 sensors, the pre-cat ones are critical for your engine to run properly. the post-cat sensors merely monitor the cat efficiency, and were installed as a result of obdII requirements. if your post-cat sensors, or even your cats are bad, it shouldn't effect the operation of your engine. they are merely for environmental reasons. so you wouldn't pass an inspection with them bad, but if you don't have time/money/desire to fix it now, you can always do it before your next inspection.

-- the above is copied and pasted from another bmw forum.
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