Dropped my truck off this morning at a local repair shop with leaky transmission lines. Shop had it "fixed" by 12:00pm so I stopped by to pay for it ($158) and get my key back. Receipt shows they replaced 3 trans lines. Wife brings me to the shop tonight to pick up the truck and the CEL is now on. So, I shut off and dropped the key back into the key drop. I'll be calling them back in the morning.

Any idea what they could have done in the process of changing trans lines to cause a CEL??