Hi Bruno, you need to be checking your oil pressure sensor wiring at the sensor you disconnect the sensor switch on the truck check voltage across +and - of sensor wiring you need 5v approx if you have that then change the sensor, if the voltage is low then you then start looking for broken wires, you have not said what truck your working on
Mod and ser/no, if it is a gas truck Mazda engine, I know that accessing the oil filter can be difficult sometimes pulling on the oil pressure sensor wires resulting in the cracking of the body of the oil sensor.