You can unplug the sensor (yes, it goes into the fuse box) and if you have a resistance substitution box, plug in values that will give the proper readings. The resistance values are found in the repair disc.
I would recommend getting the part here at MercedesShop (PartsShop or Fastlane) because the business you do here help support this site.
One thing to remember is to NOT shorten the wiring of the new sensor, the resistance is critical enough that shortening the line will affect the sensors accuracy. Wind up the excess wire and leave it in the fuse box.
Given your problem, I would assume it is a bad sensor also.