So four days ago I purchased a used 2013 A180 (W176) with 65,000 km on the clock from a Toyota dealership. The car runs fine, and while test driving or picking up the car, there was no check engine light on and everything is good. Being a dealership used car, they also provided evidence that the car has been checked and all is good etc etc etc.

Anyway yesterday (only my first drive after picking up the car) the check engine light came on out of the blue midway through my journey. The car didn't shutter or sputter or shake, and it is running fine and also doesn't seem to be in limp mode (power seems to be fine and still can rev freely with no limit). I proceeded to then take it to a car store to get a quick diagnostics done (for peace of mind because the dealer can't book me in until two weeks from now) but was not able to find any error codes - unable to confirm if this was due to the Bosch Diagnostic tool itself or what. I know the best bet is just to wait for the diagnostic I have booked in, but as this is a dealership used car there is an automatic 3 months or 5,000km warranty on the car so I wanted to make sure whatever reason they say later isn't anything to delay them from fixing the issue whilst in the warranty period.

Does anyone have any suggestions as to what this could be? Thinking back, I believe the engine light came on after the engine was restarting on eco mode so could it simply be the auxiliary battery requiring replacement? Will this cause the check engine light to come on and possibly have no error code now because it was just momentarily low in voltage? Some information say that low battery will cause the check engine light to come one, while others say there is a separate battery warning light so I require clarification on this please.

I appreciate any insights you may have. Thank you.