Circuit A refers to the power supply circuit of the sensor, which means that the ECU does not receive the correct voltage from the sensor. Maybe the ECU is out of order. Of course, the crankshaft sensor should be replaced first. It is calculated from the signals of crankshaft position sensor and camshaft position sensor. The crankshaft position sensor can know which cylinder piston is at the top dead center, and the camshaft position sensor can know which cylinder piston is in the compression stroke. When the light beam of the LED hits the photodiode, the photodiode generates a voltage. When the light beam of the LED is blocked, the voltage of the photodiode is 0. These voltage signals are shaped and amplified by the circuit and sent to the electronic control unit when the crank angle is 1 and 120. The electronic control unit calculates the engine speed and crankshaft position according to these signals.