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///M3 Forum
The BMW E46 ///M3 is the M version E46 and puts out an amazing 333 HP and 262 lb-ft of torque at stock specs! There are an amazing amount of modifications for both the coupe and convertible models so read up and get started modifying your cars today!

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Old 11-16-2012, 05:20 PM   #1
tucan709
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Join Date: Sep 2010
Location: El Monte
Posts: 22
My Ride: 2004 BMW E46 M3 SMG
02 Sensor HELP PLEASE

I truly believe something is wrong with either my 02 sensors and/or coil packs. My miles per gallon has gone down significantly in the same driving conditions (18-22 MPG, now is around 17 MPG). There is slight hesitation on acceleration and yesterday I experienced rough idling (happens sporadically, but very noticeable). And Iím telling you, when I hit a certain speed (usually 50-60MPH, the exhaust tone changes (low groan, heavy cabin noise) and the car has never sounded this way the whole 5 years Iíve owned it. I know something is not right. Something is causing this and this is my best guess.

My local indy shop helped me pull the fault codes and they were 02 sensor related and said those codes hit over 40 times before he cleared them (these codes hit without any indicator lights going on). There are pre-cat and post-cat sensors. I believe there are 4 of them @ about $185/each.

1. Front position of the exhaust manifold for cylinders 1-3
2. Rear position of the exhaust manifold for cylinders 1-3

3. Front position of the exhaust manifold for cylinders 4-6
4. Rear position of the exhaust manifold for cylinders 4-6

My question is should I replace all for preventative maintenance?

Which ones usually fail (pre-cat or post-cat)?

Which ones above are pre-cat and post-cat? (Iím assuming the front positions are the pre-cat and the rear is post-cat).

Iím afraid if I donít change them it will damage my catalytic converter. All symptoms Iím experiencing leads me to a failed 02 sensor.

Please advise. Thank you.
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Old 11-16-2012, 07:21 PM   #2
lucky_doggg7
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Join Date: Mar 2008
Location: OC
Posts: 282
My Ride: 2005 M3
Repeat after me: diagnostic work. What kind of dip sh1t mechanic pulls codes and says yup, all your O2s have given up the ghost and now you need to replace them all? This mechanic of yours, is he skilled enough to use the scan tool to use the scope, as in oscilloscope function, on the scan tool to read O2 signal voltage signals real time to see what the base line voltage is when the car is at rest (engine off), and the then see if the sensor is producing a predictable sine wave (engine on and operating in closed loop)? These are simple checks that confirm with hard testing info that indeed, a part may or may not warrant replacement.

Anyways, back to automotive 101, it is the primary, otherwise known as pre-cat O2 sensors that determine the so called fuel trim, or air-fuel ratio as it is known. A primary O2 sensor that is no longer sending sine wave signals in the recognized voltages that the ECU expects to receive, will make the ECU switch to an engine map that is meant to dial back the power (to lower exhaust temperature) to protect both the engine and the expensive catalytic converters to the degree that it works in tandem with the thermocouple temperature probe in Bank #1's cat; the ECU will work to prevent the cat(s) from overheating, and having the core melt, which will lead very bad things.

As for the post cat O2 sensors, these sensors monitor how well the cats are performing to a greater degree. Again, if this sensor gives up the ghost, the car will use another set of preset ECU maps to again protect the cats and engine.

So there you have it. Do the diagnostic work, determine which sensors are actually bad through data, not just OBD2, then proceed with repairs. Good luck fellah.
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Old 11-16-2012, 07:29 PM   #3
tucan709
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Join Date: Sep 2010
Location: El Monte
Posts: 22
My Ride: 2004 BMW E46 M3 SMG
Mr. Lucky Doggg7,

Thanks for reply. Sounds like you know what you're talking about. Well the original mechanic i took it to was an idiot. Who the hell reads fault codes, clears them then sends you on your way. I took it to a BMW M Specialist after and the codes didn't come up anymore. All symptoms point to 02 sensors but @ $200 a piece i'd rather know which one is failing as opposed to chaning them all, even though its recommended. I'll keep this thread updated.
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Old 11-16-2012, 07:36 PM   #4
lucky_doggg7
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Join Date: Mar 2008
Location: OC
Posts: 282
My Ride: 2005 M3
I should have looked at your sig. Dude, take your car to Auto Talent. It's close to you. Speak to Sam; he's the Gen'l Mgr. His mechanic, whose name I forget, is very good.
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