Dealerships
 

Can a ECM be tested before buying a new one. I have my car at a dealership and they are telling me they think this is the problem but if they install a new one and program it they can't take it back if that doesn't fix the problem.

no, an ecm or any other computer module cannot be tested. gm diagnostics are set up to rule out the wiring and controlled component/everything else. If they test ok and the fault is still there, the module is replaced. Its a process of elimination.