1999 Lesabre Security light stays on. PASS 2
 

The car cranks and runs, just the light stays on. no dtc's in pcm or bcm. I check the A3 blue wire (at module) it has 5 volts(unplug with KOEO) and 2.5v with it plugged up. It shows to be operating correctly. It doesn't have keyless Entry just 2 keys. I unplugs the connector at module, it has 12v with KOEO at A5 dark green wire, plugged up module grounds A5 and security light comes on. what should I do next? I don't have any more info on this car. Thank you

Your ignition key, has a black resistor pellet on the shaft of the key?

Yes it does.

Customer has a 2nd almost new chipped key for a spare. Same issue. Cranks and runs just the light stays on solid.

Have you checked ALL THE LIGHTS?

Hi/Lo beams, directionals, brake lights, cornering lights, perimeter lights, etc.

found 2 running lamps out, all lights working, reset the battery, security light still on

found 2 lamps out, replace both bulbs, all lamps working, performed hard reset, security light still on

The most common failure is the breaking of the wires running to the key cylinder. You can unplug the wires going up the column and check the resistance across the steering wheel side of the connector. It should be very close to the resistance across the key. If it is open, this is likely to blame, and the system luckily detected the issue while the vehicle was running and ignored the fault (very lucky).

RJolly, I am pretty sure if the wire(s) broke, it would set a P1629 code, but he says there are none....

My 1997 did that if you pulled lightly on the key after it was running. After the key was pulled out slightly for about 10 seconds it would trip the light for the rest of the drive. After 2-3 years like that the cylinder failed completely and was replaced with a new GM one. I can no longer pull the key out enough to trip the light.