P1635 GMC -

Question

2002 gmc envoy 4.2

all of the sudden I got the SES and REP lights on. pulled 3 codes originally. P1221 accelerator position sensor, p1271 throttle position sensor and p1635 5 volt reference. initially I pulled the throttle body and cleaned it but I still got the same codes. replaced the accelerator pedal and still got the same codes. replaced the throttle body and now I have p1221 and p1635 codes. pulled the fan clutch to check the wiring there for broken or chaffed wires but everything looks good. checked wiring to and from the pcm and all look good. I read one post that someone replaced their oil sending unit and that fixed the REP issue for them. they did not list if they had any codes. so I am waiting for a sending unit to replace that next.

what am I missing?