I have a 2011 C300. I recently used the heater for the first time this year. The driver side blows hot air but passenger side blows cold regardless of what setting it is on. Can someone tell me a what may be causing this issue?

 

Use a scanner that can read MB specific codes. There are many temperature sensors and you probably have a bad sensor.

 

Use the same troubleshooting procedure that AC require.
Morning & cold redout will give you calibration values.
Beware of spammers who will ask for your personal data.

 

Blend door actuator could be dead (or door is stuck).
Temp sensor for passenger side has failed and is reading hot all the time.
the driver for the blend door in the HVAC control unit may have failed.
The wiring for the sensors/actuators may have shorted or opened.

my money is on the blend door actuator.

lots of variable to just guess at. best we can do is offer what what we have seen as most common failure points. For me, i find blend door actuators to be the weak link in any system by any manufacturer.