I notice my temp gauge was low and it went back up. And then my check engine light came on. so i checked the code and it said."coolant temp,below- thermastat, regulating-temp". Does anyone kno what could be the promblem?

 

It can be a coolant temp sensor but it will be the thermostat. They both are reasonably cheap so you should replace both. The code sets when it thinks that t-stat is stuck open, causing prolonged warm-up times/open loop period where the air/fuel mixture is under an assumed calculation rather than closed loop, a real-time adjustment using the o2 sensors. Basically it's just for emissions, engine runs richer when cold, if it does not warm up within a preset limit, it will set the CEL assuming something is wrong.
And only use Mercedes brand coolant!!!

 

As MBtechinfo
I assume you have checked the coolant level.

Stuart