Q: Dealer put an "old battery" in my car, and now I'm having all sorts of problems.

Advising you is hard because consumer rights vary somewhat from state to state (I don’t know what state you are in). If you were in Ohio and they did what you are describing, the dealer would have significant "problems" both from the standpoint of what the Attorney General’s office would have to say and your own private right of recovery in, say, a small claims court. In particular, and this is a key point relative to the concern you are writing in about, the Dealer CANNOT damage your property, the glove box, and leave parts off of your car (the loose bolts), without incurring serious liability to you.

ECU’s RARELY fail. And, on those rare occasions where they do fail, studies have proven that half of ECU’s returned as bad, under warranty claims, are actually perfectly good (that’s how "rare" the actual failures are). Consequently, and not illegitimately, my "guess" would be there was nothing wrong with the ECU. You should have asked for unmistakable evidence that the ECU was bad (test and diagnostic results explained to you) before letting them replace that part. The sale to you of a used battery as a "replacement" for your battery, or even the mere suggestion that that is a legitimate repair strategy, suggests the shop is highly unprofessional.

Professional shops do not sell used car batteries, or install the same, except in an emergency. Batteries do not themselves cause short circuits, however, faulty installation of a battery will cause electrical problems and there are many ways, indeed endless ways, a careless mechanic can damage your car’s electrical system. Based on what you have described, yes, it is possible they damaged the car. If they were willing to charge $2,400 for merely an ECU, an alternator, and a $25 switch, anything is possible.

Going forward, trust but verify; that is, ask to see the diagnostic result and make sure their explanation makes sense to you and appears credible, and be sure to get your old parts back just in case you need to file a later complaint. If they are not willing to explain the diagnostic and the prices are irrational ($2,400 is irrational), obviously that is not the best shop to take your car.

I recommend having one of YourMechanic’s certified technicians come to your location to determine exactly what is wrong with your car, investigate [why the car is smoking](https://www.yourmechanic.com/services/smoke-from-engine-or-exhaust-inspection, and inspect for any electrical problems.