First time posting, upon start up my oil pressure gets up to 40-45, then rapidly drops to zero giving the " Shut Engine Off " warning and the pressure stays at 0. The engine sounds fine while it is running even with the warning to shut off the engine. That makes me think it isn't the pump. It is throwing the P0521 code. I replaced the sending unit and the filter, but it didn't change anything. It has 144k and this issue happened out of the blue while driving 60 down the road. There has been a slight variance in oil pressure between 40-50 depending on RPMs in the past, but no real loss whether the engine was hot or cold. I have looked into the issue with the o-ring on the pick-up tube as it seems to be a common occurrence on all Silverados. I have searched through these forums and haven't found anything that might be directly related to my issue. If someone could point me in the right direction, that would be greatly appreciated. Or if there is another place I should look before going through dropping the oil pan.

 

Check sending unit wiring to make sure it is not damaged. Also, I'd try to hook up an oil pressure gauge so you can tell if your oil pump is going out or not.