Testing Sensitive Automotive 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 for today's sensitive computer circuits. This current will overload will damage the control module and burn sensitive circuitry.
Use a high impedance testing device with at least ten megohms (10 million ohms of internal impedance) of impedance. It provides a very low 1.2 µA of additional load to the circuit. It protects sensitive computer circuits from the overload that a standard test light can cause.
Note that most of today's LED test lights offer such resistance and be handy in finding power. Most newer digital multimeters provide at least 10 Megohms of protection suitable for today's computerized low current automotive circuit.