Disconnect battery before removing or replacing any ECM.
Check for trouble codes.
Consult a service manual for test procedures.
Collect information on prior testing/servicing.
Verify integrity of new parts installed.
Visually inspect for physical faults.
Watch for alterations and tampering.
Check connectors of suspect faulty components.
Follow test procedures specifically.
Remember one fault can cause other faults.
Check ECM power inputs and grounds.

Connect an ECM with the battery connected.
Connect a scan tool with the engine running.
Use a jumper wire unless specified.
Apply voltage to a wire unless specified.
Alter or modify components/wiring.
Crank a fuel injected engine for extended periods.
Jump start a computer controlled vehicle.
Make substitutions of ECMs/components.
Forget about mechanical problems.
Assume diagnostic manuals to be 100% accurate.