Van won't start, anti theft security light is on. It starts but it won't hold.

Sounds like the pass key sensor is going bad. GM has an issue with these as they only last about 10-15 years. Depending on which version you have it reads a resistance from your key through and shuts down the fuel injectors. You might want to check that out as I have the same issue with a 2000 silverado and just have not purchased it to replace. Sometimes you can leave the key in the on position for about 10-15 minutes and then try to start it. You can disconnect the battery for about 10 minutes and then reconnect and try to start it. I have recently found that you can set off the security alarm while inside then use the key in the ignition to shut it off and try to start it as well. Hope this helps