I have a P0440 code in my CLK55 and cant figure out what it is.

All the vacuum lines look good, no cracking, and I changed the gas cap off my c55 and the light will still come on.

P0440 exhaust gas recirculation flow

I changed the little ticking purge valve in the driver side near the headlight.

Is your gas tank empty? Does it happen when the tank it full?

I believe the 5.5 has dual egr with sensor they are on top and in the front of the motor.

The EGR sensor/valve might be at fault, try removal and cleaning with carb spray before replacement.
Research more "dont quote me on the fix just a hunch" quote unquote
G-Luck
Gator

Small vacuum line to EGR solenoid was cracking. Replaced it and now perfect

Good to know.