OK -- Now I am getting a CEL and code 3 on the DM, which suggests something other than the O2 sensor. (I have never seen a code 3 on this car before.)

Last thing I did was spray electronics parts cleaner on the MAF sensor yesterday, and tighten down the pipe clamp at the crossover, and clear the codes at the DM. Car ran great thereafter. We had some very heavy rain today, just before this occurred -- and fuel economy dropped significantly.

The presumably-defective eBay O2 sensor is still on the car and I have a Bosch 13314 standing by for replacement. I checked the vacuum lines under the front cover and they are secure.

Since I threw an O2-related code 12 over the past few days I am assuming it is another fault with the sensor. (I am also a bit worried that I used electronics part cleaner rather than MAF cleaner on the sensor, even though that is probably not an issue.)

Any thoughts? Is it likely still just the O2 sensor?

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black