I have an 01 GT vert that I picked up about a year ago. The oil pressure gauge never worked, was always at zero. Based on previous thread, I replaced the oil sending unit this weekend. No luck, still reading at zero. What else can it be? A wiring issue? If so how do I diagnose that/fix?
First you might want to get a proper gauge on the engine (remove the factor sender and thread in a transducer) and verify that the engine oil pressure is okay.
If replacing the sender didn't fix a bad gauge reading then the problem is either in the wiring to the cluster or the cluster itself. Start tracing the wiring for the sender back. Look for areas of abrasion or damage or previous work... For example a shoddy, corroded splice or something. If the wiring checks out you may have to look into the cluster.
The previous owner may have taken the needle(s) off to slip on a white-face gauge overlay and then took them off prior to selling the car. Perhaps he was sloppy when putting the needle back on? If he really pressed on the needle to re-seat it then he may have damaged the gauge and you will need to get a replacement cluster.
Your car is +99 so can't you use your odometer to retrieve operating info like oil pressure?
If the odo area does not give you the info you need then take the cluster out, remove the clear cover, plug the open cluster back in. Next, use a salad fork to gently (very gently) lift the need off the spindle. Start the engine and put the needle on (lightly) where it should be. Turn off the engine and see if the needle touches the zero stop. If so, gently press the needle onto the spindle but don't push too hard or you will damage the gauge. If it is still off, start the engine and adjust the needle by the amount it was 'off' and see where the needle goes when the engine is off. Repeat until perfection is achieved. Keep the fork handy.
Let me add an easier way to test. Ground cluster pin #16 WH/RD wire to a known good ground. This will simulate the oil pressure sender. No need to start the car. Also this ensures the wiring and the sender can't affect the results. Easy to do since the cluster is readily avalible.