Computerized Engine Controls Diagnosis and Repair
Inputs, processing, outputs
The PCM reads sensors (MAP, MAF, TP, ECT, IAT, CKP, CMP, O2, knock) through a 5-volt reference, signal, and ground circuit, then commands injectors, coils, and actuators. Know sensor types: potentiometers (TPS, APP) sweep voltage; thermistors (ECT, IAT) change resistance with temperature; Hall-effect and inductive sensors generate speed signals; hot-wire MAF sensors report airflow. A shorted 5V reference to ground can disable every sensor on that circuit - a common trap when multiple unrelated codes appear.
Fuel trim is the diagnostic compass
Short-term fuel trim (STFT) is the instant correction; long-term (LTFT) is learned. Positive trims add fuel (lean condition); negative trims subtract (rich). Add STFT and LTFT for total correction. A vacuum leak makes trims positive at idle and normalizes at higher rpm; a weak fuel pump makes them positive under load; a leaking injector or high fuel pressure drives them negative.
Traps
A circuit code (P0120, P0340) does not prove the sensor failed - test reference voltage, ground, and wiring with a voltmeter under load. Do not replace the PCM until powers, grounds, and communication are verified. After repairs, check readiness monitors and perform relearns for crank, idle, and transmission.
Rule of thumb
If total fuel trim exceeds about +10% at idle, look for unmetered air; if it exceeds +10% only under load, look at fuel delivery or MAF. Negative trim beyond -10% points to a rich condition.