2000 Cadillac eldorado etc

Looks like the Eldorado was still using GM's VATS security system in 2000. If the ignition key has a visible chip on it, that's what you have. Is the Passkey Fault light on? If so and the engine will not crank the problem is most likely the ignition lock cylinder. There's two tiny wires inside the lock cylinder that must make contact with the chip on the ignition key. Try cleaning the ignition key first. If that doesn't work you need to unplug the electrical connection under the dash that comes from the ignition lock cylinder. It's a two wire plug, one black wire and one white wire with an orange plastic sleeve on the wires for easy identification. Now measure the resistance value of the chip on the ignition key. Insert the ignition key into the lock and measure the resistance value across the two unplugged wires coming from the ignition lock cylinder. If you get no reading or a different reading than you got from the ignition key chip, the lock cylinder is bad. Be sure to move the ignition lock through all the positions too. Look for erratic readings of resistance or no resistance reading at all. Here's where it gets a little tricky. The new ignition lock cylinder will come with a template or "dummy key" to use for cutting your new VATS keys. You MUST get the correct number VATS key blank for the vehicle to start. There were 15 possibilities. Using the resistance value you got from the original ignition key, check the chart below to determine which number key blank you need. Take that blank and the template and have the new key blank cut to match the template that came with the new ignition lock. Now, if the ignition lock checks out, you'll need to go into the climate control system and see if there's any trouble codes stored in the PCM or BCM. If the PCM has lost the fuel enable signal from the BCM the engine will not start. There will be a trouble code in the PCM to let you know this has happened but the service engine soon light will NOT come on. If this is the case, confirm that the BCM is putting out the fuel enable signal to the PCM. If it is you have a wiring problem. If not the BCM is most likely the problem. But if you don't have that code in the PCM check the BCM for trouble codes, again by using the climate control system panel. If you have codes in the BCM address those. If you don't have any trouble codes in either the PCM or BCM, replace the BCM. Or if the BCM is not sending the fuel enable signal and the wiring is good replace the BCM. With this system the BCM is plug and play with a freshly rebuilt unit. After the first start up with the new BCM installed it is "married" to the VATS key that was used from then on. This is also why you should NOT buy a used BCM. A used BCM will only allow the engine to start with the same number VATS key of the vehicle it was removed from and God only knows which one that would be!! I would strongly advise against taking this vehicle to a GM Dealer for repairs! Nobody in the service department will know anything about it, how to diagnose it, have the proper tool to diagnose it (VATS Interrogator), or how to use a VATS Interrogator! I learned that lesson with my '94 Fleetwood which uses the same security system! I wasted $700 at the Dealer and ultimately had to educate myself and fix it myself!! Hope that helps!! Jim