Most think of an ODB reader as "that thing you plug in to read the codes".

Well, most do WAY more than that. In the BOSCH 1100 series, you select "View Data" - this takes you to screens that show what EVERY sensor in the powertrain system is telling the ECU.
You can watch O2 level, throttle position, ignition timing advance, fuel trim, system voltage (as seen by the ECM, not battery voltage), engine RPM, ambient temperature, intake temperature, volume of air flowing into the engine, DOZENS of sensor readings the ECM is using to run the engine. In essence, you can know what the ECM knows - and if a sensor is lying, you will see it. (Ok, actually, timing advance is what the ECU is TELLING the timing to be - if THAT is way off, other sensors are lying to the ECM and making it make bad decisions... But MOST readings are what the sensors are telling the ECM and the ECM is using that information to try to run the engine.)

For Example:
If ambient temperature reads 172F but it's only 65F outside, you know THAT sensor is lying.
If intake temperature reads -12F, but it is 65F outside, you know the IAT sensor is lying.... etc.

If your knowledge of such systems in modern cars is sufficient, you can figure out what is going on.
If your knowledge is marginal, as mine often is, you can google anything and eventually figure out what is going on. (It will be slow going, but you will learn SO MUCH!)
If your knowledge of cars is "I put the key in and it better start"; I'd not waste my time - take the car to a trusted shop with Mercedes experience, leave them a blank check, and hope they are honest as well as capable.
......... or take the car to a dealer and get ripped off shamelessly.

In any event, you have no code or "check engine light", this tells me you have something that is not out of line enough to trigger a code OR you have something happening that IS way out, but is of nature that the ECU can't determine there is something wrong. This does not mean it is impossible to find or fix, it just means it is something that is not easily recognized by the routines in the ECU.