ECM replacement on 2001 Infiniti QX4

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My battery terminals were hooked up backwards. The 120 fuseablelink was blown. I replaced that and the car finally started but would only go 28 MPH tops. I had all other fuses checked and everything's fine. I went to Houston and bought an ECM off of the same year make and model. Same production date and everything. I was told to call an autolocksmith to come to my house and re-program my anti-theft codes. He came out and could'nt get a response from my replaced ECM.We put the old ECM back in just so I could just atleast get the car started again so I could go 28 MPH again and now my car is reset and won't start at all. Neither ECM works now. The anti-theft light is solid red ...not blinking like it was originally. Keys still work and car turns over, just won't start. If you have any suggestions PLEASE call me. Jason...281-508-8295
(1) Answer
Oh boy, this is a tough one. I would tow this (or drive at 28mph) to a good independent shop or the dealer, have them check all the diagnostic codes, record them, erase them all, and try to start again, see what codes re-appear. Fault trace accordingly. This is a problem that is extremely abnormal, and it will require a very good technician to decipher what has happened and what to do. Bite the bullet and take this to a professional.