It's not the chicken or the egg....I'll try to make this simple
As the engine runs, the several sensors gathers information and sends it to ECM, (RPM, engine load, outside air temp, RPM, injector PSI, boost PSI, throttle position, timing etc...)the ecm then process the information, and if needed makes adjustments to the engine control system using actuators. (EGR, timing control, turbo waste gate or VGT, DPF, to name a few)
If a sensor is defective, or sends data to the ECM that's out of spec, you'll have a CEL, also the failed component will cause CEL to come on. Any rate a diagnostic code will be stored in system. A tech will attach a scanner or more importantly, use mfg. diagnostic software to read real time sensor information. That will determine if information is bad (sensor defective) or component failure.....
Example- A throttle position sensor sends voltage back to ECM, this voltage varies based on where your foot is on the pedal. The ECM then makes adjustments to injection system to speed up or slow down RPM, this is verified by the crankshaft or camshaft speed sensor.....
that help?
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1999 American Eagle
ASE med/heavy certified technician
ASE advanced diesel certified
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