Can't find that code in my 98 FSM. Googled it,
P0705
Diagnostic trouble code P0705 indicates that the Engine Control Module (ECM) or Transmission Control Module (TCM) has received an input error from the Transmission Range Sensor (TRS). This switch is also known as a pressure switch, Park Neutral Position switch (PNP), the gear selection switch, or a PRNDL input switch.
That should be right on top of the transmission. Do you have a replacement?
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98 Riviera SC3800 All stock except gutted air box.
1970 Buick GS455 Stage1, TSP built 470BBB, 602HP/589TQ
Best MPH, 116.06 MPH, Best ET, 11.54
https://www.youtube.com/watch?v=UHCda-t_Jls
https://www.youtube.com/watch?v=sfT2tEO4XcU