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?