Oscilloscope

An oscilloscope is a device that displays voltage over a period of time. It displays the information on a graph with voltage on the Y axis and time on the X axis. In automotive the oscilloscope is used to measure sensors and actuator that have variable voltages that can not be seen with a multimeter like injectors, ignition pulses, MAP sensors, oxygen sensors etc. An oscilloscope can be used to diagnose engines and sensors to find faults or abnormal outputs.

Injectors

In an injector there are solenoid that is controlled by the ECU according to the engine's timing. When the solenoid receives power it opens up to allow the pressured fuel in the fuel rail to squirt into the cylinders. The  coil in the solinoid has resistance that according to ohm's law it means the volts should go down (as shown in the picture below) when the power is removed the voltage goes back up but it is noted that there is a voltage spike, a voltage spike occurs when a magnetic field collapses.

When the engine speed is increased the injectors stay open longer to run the engine richer.

Ignition

The ignition voltages are similar to that of the injectors

When the engine speed increases the number sparks also increase

Oxygen sensor

The oxygen sensor voltage oscillates up and down when the car is idling. When the engine speed is increased the voltage will oscillate slower.