I have a 2001 impala with a 3.4. The person that I got it from replaced the battery but hooked it up backwards. Now when you turn on the key nothing works at all except for the check engine light comes on. When the key is turned off the A/C button light is on. Nothing else is working. I checked all of the fuses and replaced the ECM. No change. Please help. Thank you.

 

That is the WOSRT thing that can happen to a car, it wreaks havoc. You could hook the battery up backwards and the damage can be different on every car. You will need to take it to a dealer (as much as I hate to say that) almost every module/ computer that can be destroyed must be programed when replaced. How did you replace the ECM? It has to be programed

 

Well I simply bought a used one and installed it.

 

It is not likely that that would work as it must be programmed to your car