My 2002 Yukon xl 1500 wont start.

Security light in dash remaining on now ?

Hello, the first concern is replacement of all these components without proving any of them to be the root cause of your problem. By doing that, you could possibly have complicated matters, adding more variables to an original problem. If your diagnostic tool is not communicating, check for a blown fuse in the data link connector circuit. Once you restore communication, you can check for any diagnostic trouble codes, and any key on - engine off data from the engine performance components. Vehicles need: Fuel delivery, ignition spark, the right amount of air coming in and exhaust flow leaving the engine. Along with this, you need to have appropriate ignition timing and engine compression. Beyond all, ensure you do not have an anti-theft device preventing engine operation. I reiterate: there is nothing to guess at here; it's essential you follow proper diagnostic steps, or you'll continue dumping money into something without resolve. Seek local professional guidance also for diagnostic help.