Testing Sensitive Circuits
Avoid using test lights and analog testers when testing sensitive computer circuits. There was a day before computerized vehicles when testing automotive circuits with a test light was standard procedure. These testers draw too much current through the circuit. This current will overload the control module and burn sensitive circuitry.
Use a high impedance testing device with at least 10 megohm (10 million ohms of internal impedance) of impedance. This provides a very low 1.2 µA of additional load to the circuit being tested. This protects sensitive computer circuits from the overload that can be caused by a standard test light.
Note that most of today's LED test lights offer such resistance and can be handy finding power. Also, most newer digital multimeters provide at least 10 Megohms of protection suitable for today's computerized low current automotive circuit.