Hello!  I have a 2012 GMC Acadia.  110K miles and automatic.

I bought this vehicle in 2017 and have kept it in the best shape I can afford.  

Recently it had to have the alternator replaced and the battery replaced.

It's also just passed inspection (April 2023).

Question 1 - since having the alternator replaced, I keep getting an error message "hood open".  I was told the hood latch/sensor need to be replaced, so I replaced it.  I still get the "hood open" error message....even though the codes have been cleared out and didn't show anything when being tested for the inspection.  All the electric wires look good, the connectors are tight, and the latch is securely on the radiator frame.  What could it be?  The mechanics cannot find anything.

Question 2 - Since it's over 100K miles, I want to replace the oxygen sensors.  Apparently there are 2 up front and 2 in back.

I cannot find any information on if there are 2 specific/different sensors that go up front and in back, or is it the same two sensors that go up front and in back?   The only thing I can find are sensors labeled "upstream" and "downstream"......but nothing I find states if this is just two of the same sensors, or if they are different front and rear ones.  

Can you please let me know?  

Thank you for your time, it is appreciated!  And love the videos, they have helped me a lot!!!

Thanks

Kenneth