There are a few parts in the system. There is the gauge, part of the cluster. There is a pressure sensor and the engine computer. The pressure sensor sends a signal to the engine computer, the engine computer then sends a signal to the cluster. If the gauge and the pressure sensor was replaced, then the engine is loosing pressure or the computer is going bad. They should be able to do a manual pressure test to make sure the engine is not the problem. If that checks out, then they should be able to connect a scan tool to the truck that can read engine data, there should be oil pressure data in the engine computer. They should be able to compare actual oil pressure to what the engine computer sees. This should tell them if the engine computer is getting the correct data. If the engine computer is getting good data but is telling the cluster that there is no oil pressure, then the engine computer is bad.
The way the system works is the oil pressure sensor is a on/off switch. If there is pressure the switch is on, no pressure the switch is off. The oil pressure displayed on the gauge is a program set by the engine computer or calculated, not actual readings. Here is some information from GM on how it works:
Engine Oil Pressure Gauge
The instrument panel cluster (IPC) displays the engine oil pressure as determined by the engine control module (ECM). The IPC receives a serial data message from the ECM indicating the engine oil pressure. The engine oil pressure gauge defaults to 0 psi (0 kPa) or below if:
|• ||The ECM detects a malfunction in the engine oil pressure sensor circuit.|
|• ||The IPC detects a loss of serial data communications with the ECM.|
For the 4.2L LL8, the ECM uses an engine oil switch that is open when the oil pressure is normal and closed when the oil pressure is low. At normal pressure, the engine oil pressure gauge will display 275 kPa (40 psi). At low pressure, the gauge will go to 0 kPa and the low pressure warning will be displayed in the driver information center (DIC). At high pressure, the gauge will display 551 kPa (80 psi). Any values between these three values are calculated values determined by the ECM