Check engine light is on all the time. no water in gas, gas tank is full, gas cap is on. How can I get the light to go off?

ok on todays automobiles the gas cap does more than just seal the gas tank. Now they run a return line to the tank. this does 2 things. it returns un used fuel and vapors back to the tank , and it pressurizes the tank to help improve the fuel pump.

Now as it is possible running without the gas cap may cause problems I am not 100 % positive that is your only problem.

It sound like the knock sensor is detecting the knock you mentioned and is trying to retard the timing to fix it . In doing so it has retarded it to far causing the engine to loose power and eventually stall. I would first replace the gas cap. then run a good gas additive to a full tank of gas to clean out any water and dirt that may have gotten into the tank while the cap was off. now the issue of the knock as you described. is it a knock or a ping? A knocking sound is loud metallic hammering noise. a ping is a ting not very loud. if it is a loud metallic sound I would have a reputable mechanic check it soon, as it could be a serious issue. Ping can be caused by bad or weak gas or cheap low octane gas. I would run a mid grade gas in it and see if it fixed the issue.

Try to change the gas cap first if that doesnt work check ur fuses then if still not fixed its definitely electrical

Change gas cap.

May have water in gas tank or gas lines.

If your check engine light came on shortly during or after filling the tank, and then goes off later, thats quite a relief. It really happens, and lucky for you dont have to bring your vehicle to a shop if the light persisted. Here are some tips regarding this issue: turn off the ignition switch before opening the fuel tank cap. Turn on and start only after the caps are properly closed. The scenario is that, whenever we refill gasoline on the tank, the pressure and the vacuum inside the tank are being upset, sometimes creating some kind of signal glitch which may be misinterpreted by the engine control module, and thus shortly illuminate the check engine light. I hope this lightened you up. Have a safe trip. : )

Fuel level sensor is in the gas tank, may be faulty or just dirty, make sure to fill the tank entirely full with premium unleaded, ad a high quality fuel system cleaner / de-gunker, this may clean the sensor enough to feed back to the gauge. It could also be the fuel gauge itself, but that is less likely. Mercedes will be happier with at least 1/4 tank of fuel at all times, and don't skimp on low octane gas!
As for the check engine light, you may have a bad gas cap... believe it or not a gas cap that doesn't seal properly will throw the check engine light. If you get the codes read (for free) at an Autozone or similar parts store it will likely read "evap system" as the problem, which will likely end up being the gas cap.
Take your car to a service center that does a true "flush and fill" of your a/c system. Those little canisters will just waste your money, and they never truly clean and re-lubricate the system like a flush and fill will do.

IF IT COME ON EVERY TIME YOU GET GAS.GAS CAP HAS BAD SEAL IT MORE LIKELY YOUR PROBLEM BUT LET AUTO ZONE SCAN IT FREE TO SEE WHAT TROUBLE CODE YOU ARE GETTING.

check ur gas cap first for proper seal and or not tightened this is the usual problem the evap system is from your fuel gas cap [gas--vent lines /gas tank-evap canister-throttle body] to ur throttle body/injectors if gas cap good need to check the lines and components for a leak disconnected /cracked broken-as well as the sensors and canister

code p 0440 refers to the EVAP system --purge valve malfunction
check wiring and replace the purge valve

Ok ready for this one,new gas cap,its true cars with bad gas caps will cause the check engine light go on,or your problem,it has to be pressurized when full and no air in the tank its ok,but as you use gas and have air in the tank with the low fuel the air pressure is leaking pass the cap,thus losing pressure,too bad you had to fix all the other stuff but look at it as an investment,after this works please give me a fix ya thanks