Just passed 100k got my first check engine light

After the service visit, you can post here what diagnostic code(s) got read from the car, and then we can say more about why the light was on.

Regrettably, we can't say very much just by knowing that the light was on, or even that the car runs perfectly or not.

 

that doesn't bode well...

 

Shotgun approach - NICE.... I believe the spark plugs are at or near the end of their service interval. Ask for your old parts back. You can look at the spark plugs to see how each cylinder is burning - though you won't know which plug came from which cylinder. It'll give you a clue to what to go after next if this doesn't solve your P0300 issue. It would be nice, if you just burned the tips off of your spark plugs; that way you know definitively that you solved the misfire.

IMHO: Get the ATF changed, if that hasn't already been done. It's cheap insurance, if your planning on keeping the car. Has the EGR been cleaned out.

 

If the evap (or other fuel system components) have a problem it can make the computer show you as getting better MPGs but it’s only a fictional number on the computer while in reality your MPGs actually are lower,

 

Is EGR clogging up even an issue on the 4th gen? I thought it was a redesign and not as susceptible to the 2nd and 3rd gen EGR issue.

 

The HV battery filter is easily checked. The usual maintenance is to just vacuum it from the outside at recommended intervals. I hadn't done that but pulled mine out at 50kmi and it was essentially clean, just two small pieces of fluff I picked off with my fingers. Vacuumed it anyway and put it back in. Labeled the date/miles on a piece of masking tape mounted outside above it.

Before taking to the shop I also check/clean/replace the engine and cabin filters and label similarly - most dealerships I've been to try to sell me replacements at ridiculous installation fees despite their condition. They also try to sell fuel system cleaning, suspension alignment, and various flushes despite no evidence of their need.