Chart A-1, No Mil (Service Engine Soon Light) 4L80-E W/ TCM: Notes

MIL should be on when engine is off and ignition is on. Switched battery voltage is supplied to MIL. The TCM turns light on by providing a ground path through the MIL (SERVICE ENGINE SOON) control circuit.

NOTE: Test numbers refer to numbers on diagnostic chart.

If ECM-BAT fuse is blown, see WIRING DIAGRAMS at the end of this article for complete circuit.
Using a test light connected to 12 volts, probe each of the system ground circuits to ensure a good ground is present. See PIN VOLTAGE CHARTS - 6.5L DIESEL article.