Oil pressure low 2012 Avalanche

I have a 2012 Avalanche 53,000 miles. About 6 months ago my oil sending
unit quit, changed it, & fixed the problem.  2 weeks ago my tranny quit and
had it rebuilt...$3,500. Got my vehicle back and they said my oil pressure was
going wacky and my check engine light came on when they test drove it. I get
it back and the tranny is good, but my oil pressure is running low, between 20
& 40. My oil pressure low warning comes on and check engine light. Thought
it may have been a bad oil sending unit from before when I changed it. Put a
new one in...same readings. What is going on with my engine now?