O2 sensors slow down with age.. To keep things running optimally I like to change them out every 30-40K.

I've also taken to updating our 3 wire O2's w/ 4 wire. Run the extra ground to the negative battery terminal and you'll have an O2 that responds quicker then the 3 wire.

Jonathan

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Blue Ridge Mercedes
Jonathan Hodgman
http://www.blueridgemb.com/
Enthusiast Service, Restoration & Tuning.
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Located in the Atlanta area
Specializing in all pre and post merger AMG's including Hammers and DOHC M117 engines.
Mercedes Repair Atlanta