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Message: <blockquote data-quote="lazz" data-source="post: 13329" data-attributes="member: 1214">Its is possible there has been a swap. But am wondering if it really was a Sub 2500 to start with and not rebadged like some people do. The  ECU  has the  PCM built into it so if it had a 6.0 and a 5.3 was swapped providing it had the correct reluctor on the crankshaft it would run but not well without it being reprogrammed.  Some ECU's will brick if you try it. The transmission could care less what motor is powering it. Scan and see if the VIN matches the ECU. if it matches then chances are its the original ECU. The 4.8/5.3 and 6.0 have a few differences in wiring harnesses  and most can be interchanged with little or no modification except for the trans. there are a few wires that have to be swapped to different pins. If the trans is is shifting the way you say it is then there a trans. issue. "Service Engine Soon" does not equal to "Service Transmission Soon" so transmission codes do not turn the light on. I suspect the TCC selenoid is causing a line pressure issue which in turn causes hard shifts. Also if someone installed a aftermarket Sonnex  kit in it and didnt do it correct it causes that as well. here is the test needed to figure out what is happening.12v verified at pin E on PCM to trans connector.1-2 solenoid 20ohm, A, clicks with 12v2-3 solenoid 21ohm, B,  clicks with 12vPCS, pressure control solenoid 4ohm, Force MotorTCC solenoid 10ohmTemp sensor 4k ohm.If any of these fail then check the harness to make sure its ok and replace as nessesary.</blockquote>