Originally Posted by armyinfantry3id

"The generic OBD II protocol works on a request system, which means that the scan tool has to actually ask the ECM for each PID (In OBD II a PID stands for parameter identification). This OBD II request operation contrasts with the OEM communications protocols that work using data packets, whereby, the data stream PIDs is sent in bursts or packets. However, generic OBD II standardized the whole communications process and made it possible to, at least, have access to a minimum of data for diagnostics regardless of make and model. In generic OBD II, by simply reducing the amount of PIDs on the screen a faster data rate can be obtained, since the scanner has to request less data. By combining different and faster data PIDs to form a relationship, a signal correlation can be arrived at. An example is an EGR valve that is commanded on (manually or otherwise) which should have an effect on the MAP sensor. A lack of MAP sensor response is a good indication of a defective or clogged EGR valve, since an opening EGR valve should create a drop in intake vacuum. The same PID strategy is also employed by the ECM when running each drive cycle. The difference is that we can also use these techniques to our advantage when diagnosing a vehicle."

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