Originally Posted by Just call me Sean

If your truck has a check engine light then you don't need to hook up the voltmeter, you just count the flashes. If the bulb is burned out and you don't feel like changing it, then you can use the voltmeter to read the codes. Turn on the ignition key and make sure the check engine light comes on. If you're sure you have the hookups right and they are making good contact and the computer still won't go into diagnostic mode then that usually means the computer is fried.