1999 ASTRO maf sensor

Service engine light is on, had it checked by two different mechanics. Both say Mass air flow sensor. Replaced sensor with used one from junkyard. Still bad, I guess that one could be bad but what are the odds. Is there a way I can make sure it's the sensor before I buy a new one. Can't afford to waste the money just replacing parts. Can I do a test on the old ones?
Also, gas gauge says full or empty no in between any ideas?
A\C only blows out top and bottoms vents but not front?

You will need a dvom meter to check maf or good lab scope. The vacuum line is bad going inside to ac heater controls or the vaccum swicth on control is bad. On the gas gauge sounds like bad sender.

I have a volt meter. Is there a way to check a sensor before it's put in? Also thanks on the vaccuum hose, found it was broke. Now I can't find the other end.

Wher do I find the oil pressure switch on my 2000 Astro. I think it may be the culprit in my hard to start problem?