Application: For GM vehicles from 1988 to present.
Symptoms: The P/ECM circuit boards tend to crack and disrupt current flow along the circuits inside the ECM. This can cause a multitude of drivability-related problems, such as no start, stalling, emissions failure, lacks power, and many others.
The Test: Try to get the vehicle in the state where the problem is most noticeable. Use the palm of your hand and tap on the case of the ECM. If you notice any change in the engine's condition be it good or bad, you could have a faulty ECM. Be sure to wiggle on the wires on the connector attached to the ECM to see if the problem could be caused by a loose connection. If wiggling the connectors doesn't cause a change, but tapping the ECM does, it's time for a new ECM.
It's also helpful to watch scanner data while tapping the ECM, the data will usually make drastic changes while you are tapping due to the broken circuit board.
Be careful when you are doing this test, too much force may cause a good ECM to fail. You shouldn't have to tap much harder than if you were knocking on someone's door to get the problem to surface.