Our 2010 Chevy Suburban, 80k, suddenly began running rough, mpg halved, service Stabilitrak, service traction control, and check engine light came one. When downshifting to first gear the tranny jerked badly. Every dealer gave high dollar quotes saying the problem was everything from the computer, to the cat. converter, bad lifters, bad transmission, etc etc. One of the clerks at Autozone said it was a bad coil pack. We bought one coil pack and changed out one at a time until the truck suddenly ran great. The problem was a bad coil pack on cylinder 4. Coil pack was $80 and shade tree mechanic's time was $50.