My 2009 CLK 350 had the TPMS warning come on red the other day. I checked the pressure and found the driver's rear 6 lbs. low, so I topped all of them up to 33psi and reset the system. This morning on the way to work the warning came on white, so I reset it and it came back on. I have not checked the tire pressures yet but I think I have a rear sensor failing. The back two are original I think, the fronts were changed last year when I got new front tires.
Anyways, does anyone know the mhz of the sensors I need, I've been looking on ebay and see 315 and 433mhz OEM ones?

 

Don't know the answer to your question, but check your pressures again before buying new sensors. Of course you may have picked up a nail, but a very common issue is a bad seal on the TPMS stem that leaks. Cheap to replace. My '08 sensors are still all original.

 

Tire dropped 3lbs. overnight. Didn't see a puncture, so I'm gonna order a TPMS sensor and drop it off with car for a mechanic friend to swap the sensor and figure out where it's leaking from. For $20 it's worth it to swap it just for the new seal. BTW looks like 433mhz is the correct frequency.