Based on information from various sensors, the ECM/PCM measures the fuel injection amount. The fuel injector is a solenoid-operated valve and the fuel injection amount is controlled by length of time the fuel injector is held open. The ECM/PCM controls each injector by grounding the control circuit. When the ECM/PCM energizes the injector by grounding the control circuit, the circuit voltage should be low (theoretically 0V) and the fuel is injected. When the ECM/PCM de-energizes the injector by opening the control circuit, the fuel injector is closed and circuit voltage should momentarily peak.