Our 2004 C240 Wagon has the dreaded clicking noise under the dash. All vents/functions are working as expected based on the control unit settings, except for no air coming out of the defrost vents in the upper dash.
Based on this symptom and my online research on the topic, I am pretty certain that the clicking comes from the M2/16 actuator. For that actuator there is also no plastic lever/arm and just a pretty solid plastic "key adapter" between the flap and the actuator, so it is probably unlikely that this is broken.
I also did the reset/test by pressing and holding the record and defrost buttons until their LEDs start flashing back and forth. The test never finishes, indicating that something isn't right.
However, to my surprise, during the test cycle there are times where there is full air flow through the defrost vents, so the defrost flap is successfully being opened and closed by the actuator.

My dealer stated that "“it is entirely possible that it just needs to be taught-in” via the Mercedes diagnostic/programming tools.
Before paying the dealer for diagnostic/programming work and/or me starting to take the entire dash apart to replace the actuator (and probably all the plastic key adapter), I wanted to see if anybody here has any information on the "teaching" or the situation in general.
Is this a thing for the ventilation system and could this be actually all that is required to fix this?

Any help and concrete information on this is greatly appreciated.

Thank you!

C240 actuator position, function and labels


"key adapter" between the defrost flap and the actuator - beige plastic thingy in center (screenshot from online video)



Location of M2/16 actuator - lower left corner (screenshot from online video)

 

If you haven't already done so, take a look at the HVAC stepper motor sticky.

Looks like you already did the super hard part of taking apart the entire dash. One thing I would also do is open up the motors and look for broken teeth. If the gears look worn, just replace the entire motor. Not worth tearing up the dash again, and the defroster motor is by far the most difficult one to access. Unfortunately the internal gears can't be purchased separately.

Are you sure it's the defroster motor that is broken? Run the calibration test again with the dash off. You'll find your answer real quick. Keep in mind that a broken gear might still intermittently engage. You'll have to fiddle with controls and catch the failure in action to be 100% sure. Don't just throw parts at the car.

While you have everything part, check the other motors. Anecdotally, the motor that seems to die first (most common problem in the forum, and also can personally confirm) is actually M2/15 (directs airflow to the footwell). Personally I found this motor died well before the next one. Guessing the system "drives" this motor harder or more often than the others. This motor is very close to the defroster motor, which is why I suspect you may be looking at the wrong motor. FYI the footwell motor failure thump noise comes from the upper part of the L edge of the radio when the dash isn't taken apart.

Not only does the footwell air motor tend to die earliest, but the original undersized linkages for that motor are known for breaking at the hex end (basically 100% failure rate). If you bought your car used, there is a good chance a previous owner got this done (possibly under warranty), as they normally die well before 100k km. There is an updated PN A203-830-00-33 (contains both L and R arms) where they made the hex ends thicker. Once again, while you have everything taken apart, I would definitely replace these if you are still on the older version. Not sure if you have the dual zone option, but if you have a single zone, you might be able to just purchase a singular L arm (which controls both zones) separately, though I imagine the cost difference would be minimal.

The motors can also be opened up. The internal gear teeth do wear out and strip, but it's not always the same gear that breaks. I would keep your old motor assemblies and save the "good" gears and electronics for future replacements. Personally I've had 3 motors fail, and each time it was a different internal component that failed. 2 times it was a (different) gear, and the 3rd time the actual electric motor died. I was able to Frankenstein a good "complete" motor assembly from the remaining good parts.

Anecdotally, the sounds I found were these:

Failed footwell linkage - sharp snapping sound (like someone breaking a brittle piece of plastic), about 1s apart - fixed under warranty so I didn't look much into this, and they only replaced the linkages but not the motor
Failed footwell motor - dull thumping noise, about 1s apart (this is what I suspect you are actually facing) - caused by internal black gear wearing out and then skipping teeth instead of engaging
Failed defroster motor - no sound - caused by a tooth completely breaking off the large white gear, so the other gears were just spinning. However they would also engage intermittently.
Failed fresh air (blows towards face) motor - sharp ticking noise, about 5 per second. Caused by the actual electronics failing (might've been a bad gear but I couldn't find anything). Electronics are not serviceable at all without destroying the metal casing.

 

Replece the broken stepper motor and you will be fine.

 

To answer your question, sort of. You hold...uh the defrost and....maybe the recir button down with the key in the 2nd position?
It's a matter of bushing and hold 2 of the buttons at once till it starts flashing.
If it completes, it will stop. If it fails it with run continuously. I don't have time to look for it,
but if you search you should find. A scanner will tell you which flaps are stuck, actuators bad.
I had iCarsoft and I get errors on the climate control system and the angle of the flaps even.
So you can see it they are moving in live data.
I have air blowing on the floor always, so thats my failure.