PIDS are the serial data that can be accessed from the vehicle computer using a scan tool. PIDS include:
PIDS provide valuable diagnostic information when checking the operation or status of various sensors, circuits and switches in the vehicle's engine management system.
For example, if the MIL lamp is on and you find an oxygen sensor code, you can call up the oxygen sensor PIDS on your scan tool display to see what the oxygen sensor is telling the PCM.
You can also compare PIDS to see how one component may be affecting another.
For example, when you suddenly open the throttle on an idling engine, rpm should increase, the TPS reading should change and the MAP sensor value should drop.
PIDS can also be compared using a "graphing multimeter" or on a scope that converts the voltage values to waveforms.
Comparing the waveforms of several related sensors can help you find faults that might otherwise be impossible to detect.
Different scan tools have different capabilities to display PIDS.
The OEM scan tools used by new car dealers are capable of displaying every possible PID value that is built into the engine management system.
Most general purpose aftermarket scan tools do not contain the software that allows them to match the OEM scan tools in every respect -- but for most applications they can display all the important PIDS.
The trouble is you never know what PIDS are missing until you go looking for one and find it isn't there. Bummer.
That's why many professional technicians own multiple scan tools: an aftermarket general purpose scan tool, and one or more OEM scan tools for the makes they most frequently work on.
Scan tools like TOAD support various PIDS including live data values, the status of switches and other devices, the readiness status of various OBD II monitors, and other test results.
Live data provides real-time measurements of system inputs.
Statuses tell you if a switch, relay or other device is ON/OFF or has been commanded ON or OFF.
Readiness monitors tell you if the monitors have completed or not.
Test results are measured by the PCM and compared against preprogrammed pass/fail values in teh PCM's memory.
For wide-band O2 sensors and linear O2 sensors, the value may be higher, or it may be converted to a zero to one volt scale.
To obtain the actual air/fuel ratio being commanded, multiply the stoichiometric A/F ratio by the equivalence ratio. For example, gasoline, stoichiometric is 14.64:1 ratio.
This is an important value o check if the engine has lower than normal LTFT and STFT fuel trim numbers (indicating a rich fuel condition). The purge valve may be leaking vapor into the intake manifold.
Code readers and scan tools will also display Stored Diagnostic Trouble Codes (DTCs), usually in numeric order.
Many scan tools can also display Pending Trouble Codes.
These are codes that indicate a fault has been detected, but that the fault has not yet repeated.
If the fault repeats under similar driving conditions, it will usually cause the Pending Code to become a Stored Code and turn on the MIL light.
Many scan tools can also display Freeze Frame data.
These are PIDS that are captured when a fault occurs so you can refer to them later when diagnosing the problem.
Freeze frame data typically includes related sensor values at the time the fault occurred.
OBD II requires the following status and readiness monitors:
