The oil pressure light in a 2000 GMC Sierra is especially serious. It means your engine isn't getting sufficient lubrication, leading to catastrophic engine failure if driven. Do not drive the vehicle if the oil pressure light is on.
Here's what you should do:
1. Check the oil level immediately. Use the dipstick. If the oil is low, add the correct type and amount of oil as specified in your owner's manual. But even if you add oil, you still need to diagnose the cause of the low pressure! Driving with low oil level damages the engine even further.
2. Check the oil pressure. This requires a mechanical oil pressure gauge, not just the light on the dash. You can borrow or rent one from an auto parts store. This will tell you definitively if you have oil pressure.
3. If the oil level is low or you have no/low oil pressure: Have the vehicle towed to a qualified mechanic immediately. Driving with low oil pressure will destroy your engine. The problem could be something simple (like a faulty oil pressure sensor), but it could also be something severe (like a failing oil pump).
4. If the oil level is correct and you have oil pressure (confirmed by a gauge, not just the light): Then the problem is likely with the oil pressure sensor. While it's possible to replace this yourself, it's still best to have a mechanic diagnose the issue and make the repair. A failing oil pressure sensor could indicate other problems.
The check engine light is separate from the oil pressure light. Once the oil pressure issue is resolved, *the oil pressure light should go off*. The check engine light might still be on, and requires a diagnostic scan to determine the underlying code. You can use a code reader (available at auto parts stores) or have a mechanic scan it.
In short: Focus on the oil pressure light first. It's the far more critical warning. The check engine light is important, but engine failure from lack of lubrication is much more devastating and expensive.
