I hooked up my OBD2 reader and got P0451, which a problem with some evaporative pressure sensor. Car is a 2015 ML350 with about 60k miles and about 6 more months of CPO warranty so... is this kind of thing a warranty issue? (I'll take it to the dealer later this week, have to wait for my other car to come out of the shop first). I've read that it can be challenging to trouble shoot, which has me gritting my teeth and clutching my checkbook tight. Just looking for a little background info before talking to Mr Dealer Man.

Thanks!
KJ

This is what I found on the internet:
This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles (Dodge, Ram, Ford, GMC, Chevrolet, VW, Audi, Toyota, etc.). Although generic, the specific repair steps may vary depending on make/model. If your OBD-II vehicle has presented a code P0451, it simply means that the powertrain control module (PCM) has detected a malfunction in the pressure sensor circuit of the evaporative emissions (EVAP) system. The EVAP system utilizes a vented reservoir (usually called a canister) to store excessive fuel vapors until the engine is being operated under the appropriate conditions to burn them thoroughly. It is designed to capture fuel vapors before they escape into the atmosphere. The charcoal canister, the EVAP pressure sensor, the purge valve/solenoid, the vent control valve/solenoid, and a complex system of metal tubes and rubber hoses that stretches from the fuel tank to the engine compartment are all crucial to operation of the EVAP system. Fuel vapor pressure, created when fuel is stored, acts as the propellant and causes the vapors to vent through a network of metal tubes and rubber hoses; eventually reaching the canister. Fuel storage tank vapors are discharged through the pressure relief valve (in the top of the fuel tank) and forced through a complex web of metal and rubber lines to the charcoal canister, where they accumulate. The canister contains a charcoal element which absorbs fuel vapors and holds them for release at the appropriate time. The purge control valve/solenoid is responsible for regulating intake vacuum to the EVAP canister so that fuel vapors may be drawn into the engine when conditions are ideal for them to be burned instead of being released into the atmosphere. The PCM electronically controls the purge control valve/solenoid which is the heart of the EVAP system. EVAP system/fuel tank pressure is monitored by the PCM using the EVAP pressure sensor. Located in the fuel tank and integrated into the fuel pump/fuel level sending unit housing, the EVAP pressure sensor can be quite challenging to access. If the PCM detects that EVAP and/or fuel tank pressure is not within a pre programmed range, over a set period of time and under certain circumstances, a code P0451 will be stored and the malfunction indicator lamp (MIL) may be illuminated.

Read more at: https://www.obd-codes.com/p0451
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This happened to both our '14 ML350 and '14 GL450. I got a check engine light in each a few weeks apart. In both cases I removed the gas cap while the car was not running and put it back on, turning it 4 or more clicks. Cap says turn past 3 clicks. The check engine light went out the next day after a few shut downs and start ups. Conclusion? I was lazy (twice) about the 3+ clicks after getting gas. Now I turn at least 5. Hope it is as simple for you.

 

I'm pretty all sure that's a different trouble code because my wife has done that more than once (one of the reasons I was prompted to get the reader in the first place.)

That would be a pleasant surprise though.