GF was driving my car, she shows up to collect me from school in it and the yellow check engine light, with a lightning bolt on top of it was on. She says she never noticed it come on...

I checked all the fluids everything is good, by the cluster temps and voltages are good. battery shows 12v when key is "on" and 14v when running so Alt is working.

Any ideas?

 

That is just about impossible to figure out why the check engine light came on. I'm guessing you don't have a vag com or any code reader. My suggestion is go to advance auto parts or autozone tell them ur check engine is on and they will diagnose your car and scan for codes. Could be even the smallest thing like loose gas cap But without codes it's impossile to tell.

 

^^^^^^^

100% this

the light means that your ECU has registered a trouble code. the only way to know which trouble code is to hook up a scan tool and retrieve the code. if you go to a chain auto parts store they will be able to retrieve the code, but may not be able to tell you what it means. write the code down and post it back here and i'm sure someone can point you in the right direction.

 

Yup.. trouble code. Could be a misfire/bad coil or possibly a spend O2 sensor.. a couple of the more common ones.

 

Alright I did just that...

Scanner Test revealed one code: P0173 - Fuel Trim Bank 2.
We later found a hole in the flex pipe of the passenger side exhaust line and exhaust gas was leaking out. Mechanic suggests fixing the leak will fix the problem as it's probably due to one O2 sensor reading differently than the other side (Bank 1).

What do you guys think?

Should I replace the flex pipe? They reset the code and it hasn't come back on since and I drove for the evening and it drove fine still.

Are there any aftermarket exhaust options worth considering for that section? - apparently to unbolt it at the upper flange its right attached to the headers which means the Axles would need to come off to gain access to them.

Also, the front calipers are seizing up a bit and don't actuate as quickly as I'd like so I'm considering having them replaced with rebuilt calipers for 135$ each. What do you guys think? are there any worth while upgrades or substitutes for oem?

 

Drive the car around a good 100 miles and the MIL light should be back on. U just cleared the code not fix the problem.