How to find shorted to high/ low source voltage ?

[Rawze]

Some sensors when they are unplugged will report as "high" out of range. Others will report as being "low" out of range. This is because some sensor circuits in the ECM have a resistor on the pin to "pull up" the voltage if it is unplugged, while others have a resistor to "pull down" the voltage to ground. Understanding this is the first step for any given sensor.

When the sensor is NOT giving any kind of fault, if you unplug it, which code does it throw?. Shorted "high" or shorted "low"?. - Knowing which fault occurs when it is simply unplugged will tell you if it is seeing a broken wire or not. If the sensor if throwing the same exact code when it acts up than it does when it is unplugged, then is is a good bet that perhaps the wire has a break or corrosion in it somewhere. Otherwise, if it is throwing a different code, then perhaps it is a short or moisture in a plug or harness causing it.

btw: What sernsor? What engine model? What kind of truck/year/miles, etc? and what are the conditions/history of how it acts up?