My wife's 850 turbo was recently at a body shop for about a week to get the rear bumper replaced from a minor accident. When we went to pick up the car, she started the car up and there was a lot of white smoke coming out the exhaust and the valves were tapping pretty loudly. The idle was pretty rough too. The tapping was so loud that even the guy at the body shop opened the hood to see what was going on. Anyways, we were all surprised because we didn't think they can mess up the engine via fixing a rear bumper. So we decided to drive the car home. But after one block, the white smoke was so pronounced and the tapping so loud that we took the car back to the shop. The guy told us it couldn't have been them that caused this because they never even did anything to the engine. They said that all they did was disconnect the battery to replace a tail light assembly that had been damaged along with the bumper. Apparently the battery disconnect also triggered some codes so we need to have the dealer correct them. Even the ABS light came on.

I'm wondering if messin up the codes can create problems for the engine. If not, does anybody know why the engine would run so poorly? At this point, we are at a loss because the shop is not claiming responsibility, and quite frankly, I'm baffled at how fixing a bumper can mess up the engine. The guy also said it could be condensation in the cylinders from the car sitting for a while and them only driving it for a few seconds at a time to move the car from one part of the lot to the other. Is this a plausible explanation?

Thanks for any insights.