My 1988 260e is failing the emissions test and people tell me that its' probably a bad oxygen sensor. New ones (bosch) go for anywhere from $170.00 to $116.00, but I found an aftermarket one (walker) for $39.95. (thats right, only $39.95..) Does anyone have any experience with these ? Is there a way to check those out to see if its' bad ? Thanks.