OK, I'm losing gas mileage in my 2000 ML320 (almost 100K miles). I've read that the oxygen sensors may be the reason. I've removed and "cleaned" the MAF sensor (spray MAF sensor cleaner) with no change to mileage. Will changing the "front" oxygen sensors make any difference? (Do I have to do all 4?). If so, is there really any deal in buying them?

Cleaning the rear O2 sensors will not help, as those play no roll in the mixture of the motor, but simply monitor if the cats are functioning properly. If any O2 sensors will have an effect, it would be the fronts. However, I would change plugs before trying to gain mileage with O2 sensors. Also, checked your air filter lately? That is often an unsuspected culprit. Go with OEM plugs, not +2s or +4s, as those often do more harm than good.

The spark plugs were changed around 10K miles ago (with OEM-like style). The air filter is still good. You said "cleaning the 02 sensors"... is that an option rather than replacing them?

Sometimes you can clean them, but really does not prolong their life, just makes 'em work if they got gunked up due to a problem upstream. Unless they are new, cleaning is really a waste of time and effort. I still doubt O2's are the problem...checked your allignment lately? Truck going straight down the road? That can also be a way to lose mileage.

I have the same year truck and the same issues. I also get codes 300, 301, 302, 303 & PO 130 for bank 1 sensor 1. I found O2 sensors from $44 www.*********************** for universal OE replacement (cut and splice connector) to $95 for Bosch OEM drop in. The question I have is it suggested to replace both front if only getting a code on one. I'm at 102k right now and milage has dropped 3 mpg in town.

same problem and same codes on my 01 with 90K. i was told changing was very difficult job and best left up to the prows. is this true or is it somthing a weekend michanic can do?

I have been getting a intermittant check engine light . It came on yesterday and I took it to my mechanic, he used his scan tool to tell me that both rear oxygen sensors were showing to be a problem . He then said he needed to check the relay and voltage to them to make sure that they were the problem. I have 135000 kilometers (90000 miles) on the car and they are the original sensors. The o2 sensores from Mercedes are ridiculously expensive but I check the link above and 44 dollars for splice on sounds good. My question...can I buy a tool to get my own codes and would you recommend the universal splice on oxygen sensors and also do you think it is worth going back for further diagnostics or just replace them?