so took my car in for the CEL
thy said they need to replace both front sensors.
does the car have rear sensors too?

Pre cat AKA front and post cat AKA rear

so 4...ok

Yes.

found my papers from last year, it was my right front sensor that caused CEL....why the hell does this keep happening

I'd ask your service manager but my Audi dealer suggested using Shell V-Power premium gas after an O2 sensor replacement in my wife's car. Since we switched all of our cars over to Shell, we have not had any problems with the emissions systems in our cars.

Ferrari O2 sensors don't like much other than Shell on a regular basis for some reason. I use Exxon or Chevron in the German stuff, but recent Jag didn't do well with seasonal blend changes at either, did fine with Shell.

i dont have a ferrari

ive been using chevron for 20 yrs....i dont think it was the gas...i think mercs has crap sensors.......my opinion seeing that there really is no good answer

Christ, not literally.
I've had a Ferrari that had O2 sensors fail on Chevron, but they're fine on Shell.
Example of O2 sens failure needing Shell. Not unknown. Warranty paid for it.