How do I check no oil pressure on gauge? The oil pan is full.

check the oil pressure first, it should read around 10 to 14 psi. If pressure is inside the range then your pressure sensor is bad. If lower or reads 0 psi your got a engine problem, further diagnosis needed. Good luck.

Could be the gauge on the dash. If you really had no oil pressure the engine would be very loud.
A mechanic would check oil pressure with a mechanical gauge to verify.

If the gauge is showing zero pressure, you would next check the pressure with a mechanical gauge to verify.
Make sure you use a tee fitting to keep the electric sensor in use while testing.
If the pressure really is zero, something has happened to the oil pump in the oil pan.

The oil pressure should be verified by removing the oil pressure sending unit and installing a gauge to take a direct reading. If the oil pressure is ok, the problem is most likely a bad pressure sensor (although it could also be a bad gauge or circuits).

If the gauge verifies that the oil pressure is indeed low, then the oil pan needs to come of for an inspection of the oil pick-up tube and the engine bearings.

Sound like a posible bad oil pressure sending unit . It is normal for pressure to be above normal just after starting . At a cold idle , the oil viscosity is higher than it would be if it were warm . If the guage is reading really high for longer than normal ( after 1 minute of cold idle ) then I suggest having the sending unit checked .A sevice center can check the sending unit on or off the vehicle as well as checking pressure with an actual guage for more accurate readings .

Sounds like you have a bad oil pressure sending unit, the oil pressure sending unit controls your oil pressure gauge, what I would do is remove the oil pressure sender and hook a manual oil pressure gauge up to your engine and verify you have sufficient oil pressure, if you have good oil pressure on your gauge then replace the sending unit, if you have a low reading on the gauge stop your engine immediatly and start looking at a potenial problem with your oil pump or lubricating system within the engine....

The first stop is to measure the pressure with an external gauge. if the readings are the same, surgery is needed, I would look at the oil pressure relief valve for foreign particles.

It takes resistance to make pressure, and your decsription does not sound good. If the pressure is actually dropping, and you find no issues with the oil pump and pick up, you need to measure bearing clearances... Essentially, everywhere oil is pumped needs to be inspected...

If you got the same oil pressure reading with the new and old sending unit, more then likely your oil pump pick up tube is clog up, remove oil pan and clean or replace pick up tube also replace pan gasket.Goodluck.

At this point, don't worry about the A/C...check to see if the wire is connected to the sending unit for your oil pressure. If it is, then, does the engine have any knocking or tapping? If it is, don't drive it. If Its still quiet then replace the sending unit.
I see no reason for any connection between the two problems but you never know what some crazy engineer may have designed into the system. Protect the engine first, then if necessary diagnose the a/c problem separately (could be a fuse, relay or wiring defect)

i had same problem and its just oil pressure sending unit its above oil filter just check it if you got some loose wire or somthing .........my problem was ;i had oil in conector i clean it and works jyst fine