Help im new
 

:confused: i just joined today i bought a 2000 cadillac sts 2 weeks ago i bought it with some problems in it checkengine light is on and some times it shakes and bad bad bad gas mileage the computer reads 15 mph avg the onwer said it needs coils i fixed the front coils my self but still i have a promblem and it still shakes i need help.

1-can i replace the back coils with front coils does it matter ?

2-how can i fix the gas mileage and if anyone knows how to fix any prolems i just mentioned?

Coil packs are left and right, What codes are stored?