P1258 code Chev 6.0 L96 engine

Just got this donor engine (chev 2012 6.0 L96) running in a 1983 G-30 Van. Engine fault codes (see attached screen shot of errors, [engine not running]) I need help, any suggestions. Have double checked all associated wiring most of the harness is from the donor vehicle. The exhaust is open (high noise) just aft of the first set of (forward) 02 sensors, back sensors are open (not installed yet). The P1258 code is (I think putting the engine in limp mode). Checked the temp sensor wiring to the computer, reviewed the specs on the sensor all checks good. Installed Dakota VHX gauges, the temperature reading is taken off the OBD2 buss is giving good numbers, so it appears the sensor is good. Yet am getting P1258! I will admit am on a very steep learning curve. Also checked the throttle wiring, all checked out as per GM manual. This engine has been setting for about 3 years. When purchased was told that the engine ran very well and had no known problems. The body computer was not used. The only computers utilized are the fuel, and the transmission (6L90E six speed) computers. I have also attach codes with the engine running. Also, the fuel tank pressure and associated sensors and not in the system anymore. that wiring was capped and stowed.

Again if anyone can give me some direction I'd appreciate it.

Russ C

Has the PCM been flashed for "Stand-Alone"?

Maybe post the flash for a second opinion?
Mosttimes a parameter needs to be set to "No Error Reporting" or set a value of maximum MPH / Temperature for Stand-Alone to work correctly.

It's also a good plan to have all the sensors connected when running the engine to prevent a cascade of errors.

dave w