You know, I did buy a basic CCP Vag-Com reader (emulator) online. No PID's but airbag/ABS.

The Vag-Com code descriptions in the provided booklet gave a broader base of culprits then the OBD code descriptions. One fix from the el-cheap-o Vag-Com paid for it AND the expanded OBD with PID reader.

Specific issue was the ever present VW coolant temp sensor code. Booklet said could be thermo housing and/or thermostat also so I replaced both. Not real scientific but a better clue than with what OBDII codes gave me.

CAI and airbox are before the MAF.
Your airbox probably did not trigger the CEL.
Read the stored codes

Yepper. Betcha gas cap or some evap code.

Stored codes are gone if battery disconnected as OP said, unless some snazzy scanner can pick them up.

The Mercs use a different diagnostic system, SDS. Last time I used it, found the speed sensor was the culprit for the ABS light. Was interesting graphing out the fuel trim on the Accord as well and seeing the adjustments made at different running conditions., But yes, one fix will pay for the unit especially on anything of German make !

Thanks. I’ll do it. I didn’t think the airbus leak would trigger the CEL. I’m hoping the gas cap gasket is the culprit.

Typically it's something emission related. You could have an upstream or downstream o2 sensor on the fritz, could be a cylinder misfire that popped up and went away. When you check the codes report back.

I've driven cars with the CEL on knowing that it was a P0420 code. I got tired of looking at the light on my Taurus and finally replaced the catalytic converter that was pretty much shot. My Subie, I tried to fix the problem a few times and just gave up. It didn't hurt mileage or performance so it became a mind over matter thing, if I don't mind, it don't matter.