Lab Scope Patterns

A scope is only useful if you know what the waveform is supposed to look like. Every healthy component produces a specific pattern. Every type of failure distorts that pattern in a specific way. Learn the good pattern first. Then the bad patterns become obvious.

A good primary ignition waveform has three distinct sections. The firing line — a sharp vertical spike when the spark plug fires. This spike represents the voltage needed to jump the gap. On a healthy plug with a proper gap, this spike reaches a consistent height. If the firing line is too tall, the gap is too wide or the plug is worn. If it is too short, the plug may be fouled or the gap too tight. After the firing line comes the spark line — a mostly horizontal line that represents the duration of the spark. It should be steady and level. A spark line that bounces or is jagged indicates turbulence in the combustion chamber or a plug that is breaking down. After the spark line come the coil oscillations — a series of diminishing waves as the coil energy dissipates. Clean, even oscillations mean a healthy coil. Oscillations that are uneven or dampened quickly can indicate a failing coil.

When the PCM commands an injector open, you see a sharp voltage drop as the solenoid is energized. The injector opens and fuel flows. When the PCM releases the injector, the voltage spikes sharply — this is the inductive kick from the collapsing magnetic field. Right after that spike, look for the pintle hump — a small bump in the waveform caused by the injector pintle physically closing and bouncing. A clean pintle hump means the injector is seating properly. A missing or distorted pintle hump means the injector is sticking, worn, or contaminated. Compare all injector waveforms side by side — they should look nearly identical.

A magnetic crank sensor produces an AC sine wave as the reluctor teeth pass by the sensor. The pattern is a series of evenly spaced peaks — one for each tooth. But there is a gap. The reluctor wheel has one or two missing teeth that create a larger gap in the pattern. The PCM uses this gap to identify crankshaft position. If the gap is in the wrong location or the peaks are uneven in height, the reluctor wheel is damaged or the sensor air gap is incorrect. A clean crank pattern with consistent peak heights and a clear missing-tooth gap is a healthy sensor.

A healthy conventional O2 sensor on a warm engine switches rapidly between approximately 0.1 volts (lean) and 0.9 volts (rich). It should cross the 0.45-volt midpoint at least six to eight times per ten seconds. That is good cross-count — the sensor is responding quickly to changes in exhaust oxygen content. A lazy O2 sensor switches slowly — big, slow, rounded waves instead of sharp, fast switching. A dead O2 sensor flatlines at one voltage and never moves. The scope shows you the difference between a sensor that is responsive and one that is sluggish in a way that PID data on the scan tool cannot match.