1995 gmc yukon when the motor is cold the oil sending switch show a psi of 40 when the motor heats up the psi goes down to 20 when you are either at a stop or idling when you are going you see the sending switch 40 to 60 psi how do i test the sending switch to see if it bad i dont think it the pump becuse the oil is right on just did an oil change about 2 month ago this just started any help please

It means that you need a mechancial oil pressure gauge to test the oil pressure and if it shows the same thing it means when the engine warms up the crank shaft bearings or the cam bearings or rod bearings are worn out.You could put in STP and see if it rases the pressure. But to test the pressuer swith you would have to apply a known amount of pressure of air and mesure the resistance as it goes up. Even then it would be faster to just replace the switch, if the oil switch is on the back of the block it will get to the point that it will not start. so just replace it it's $15.00 and if it still does it you know what your facing.

You may have a bad oil filter, the best check is to install an oilpressure guage to find out what the actual pressure is, or maybe you put in too light an an oil,