The sender is rarely the issue here...I've also replaced my cable hoping to rectify the same problem (reads about 11 degrees lower than actual, based on the readings I get on my other MBs).

The display behind the cluster is the culprit. Unfortunately, an OEM replacement is pricey ($200+). I've read threads where you can obtain a replacement thermistor from Radio Shack for about $5 that will solve your problem. Not sure what it replaces or how to install though.

Do a search on "outside temp gauge"...there might be more info.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car