i have a 99 atro 4.3. lately it has been running rough but now it will start for only 2 seconds then shut off. i replaced the cam position sensor and didnt fix it. i pulled codes p0108,p0144,p0155, and p0303. i noticed a vacuum line or two not hooked up and dryrotted, dont know if this could be a reason. any ideas????? thanks

 

A vacuum leak can certainly contribute to "running rough" especially at idle but I'm not sure about "2 seconds then shut off" part. Is the SECURITY light in the I/P cluster lit or flashing with key on?

As for the trouble codes, how long has the CEL been on? I'm surprised that they are all over the place:

P0108 - MAP Sensor Circuit - High Input
P0144 - O2S Circuit-High Voltage (Bank 1, Sensor 3)
P0155 - O2S Heater Circuit Malfunction (Bank 2, Sensor 1)
P0303 - Cylinder 3 Misfire Detected

 

security light was on last time it ran.

 

The SECURITY light in the I/P cluster is lit constantly with key on, is that it? Check the SECURITY fuse in the I/P fuse block and make sure there's battery voltage present on both sides of the fuse with the fuse in place. The fuse has probe holes for taking voltage measurements. If it checks out, you probably have a faulty Passlock module (not to be confused with the Passlock sensor on the ignition switch) or wiring to and from. Don't waste your time on trying to get it to relearn which only works when the SECURITY light flashes. Consider towing it in.