It doesn't have to be running to use a scan tool on it, and even if it's not running, scan data can be very useful to determine WHY it's not running.
The common issues with the electrical system are the relays, they are junky for some reason whether for the headlights, blower, or whatever. There are also a couple of known areas where wiring chafes and shorts out.
In your case, however, I would start with basics, as always. Test fuel pressure, test to see if there is spark. Fuel pumps are fairly common failure, and so are ignition modules, so either of these things would cause the car to not start. If the scan data reveals 0 rpm when cranking, you'd need to look at the crank position sensor, another fairly common failure.
Good diagnostics are what is needed here, despite the fact that Catera is a different kind of animal, all of the same basic rules apply: fuel, spark, and air.