Computerized Powertrain Controls Diagnosis (including OBD II)
Inputs, processing, outputs
The PCM supplies a 5-volt reference, reads a signal return, and completes the circuit through a dedicated sensor ground. Know the difference between a two-wire variable resistor (ECT, IAT), a three-wire potentiometer (TPS, APP), and a frequency or digital sensor (MAF, digital MAP). Never probe a 5-volt reference with an incandescent test light; it can pull enough current to damage the driver.
Scan tool literacy
Understand freeze frame, readiness monitors, Mode $06, and bidirectional controls. Scan tool data can mislead: a sensor with a skewed ground reads plausibly but wrongly, so cross-check with a DMM at the sensor and at the PCM.
Fuel trim as a diagnostic tool
Short-term trim is the fast correction; long-term trim is learned. Positive long-term trim beyond about +10% means the PCM is adding fuel: vacuum leak, low fuel pressure, or a MAF under-reporting. Negative trim means it is removing fuel: leaking injector, high fuel pressure, or a MAF over-reporting.
Traps
Diagnosing an O2 heater code as a mixture problem; blaming a no-communication fault on the PCM when it is a blown fuse, bad ground, or shorted data bus; forgetting that clearing codes wipes adaptive learning and resets monitors; and ignoring a TSB or calibration update.
Worked example
A TPS code with a 0.4 V glitch: backprobe signal and ground, sweep slowly, and watch for a dropout. A dropout at one point is a worn resistive track, so replace the sensor. A smooth but offset reading means the reference or ground circuit is at fault, not the sensor.