This may be a simple fix, but I've exhausted all options and I am stuck.

Vehicle: 05 Impala LS 3.8L

***Replaced radiator.*** Battery and computer both were disconnected when it was replaced, obviously. Car started fine before the replacement. Now it will crank and then die almost immediately (within 2 seconds). Tried all the relearning available and used timers. Hood and doors shut, etc. No fuel pressure. Don't know what difference that's making, because again, it cranked before everything was unhooked. Tried EVERY OPTION. Any help appreciated. Thanks!

 

such as? Please list everything you tried so we're not wasting time.
Obviously no fuel pressure is a show stopper, so what have you tried in that regard?

is the security light lit on your instrument panel? Any other warning lights?

 

So, really just don't know where to go from here. The car ran, replaced the radiator, now no start.
Checked fuel pressure, and nothing. Have charged the battery thinking it may have dropped below the required 9V for the computer to work correctly.
The "relearn" consisted of sitting in the car, every door shut, hood down, and only window down was the driver door. Turned the key to "on" and waited 10 mins (used a timer), then "off" for 5-10 seconds, then proceeded to try to crank. Done this a total of 5 times. No success. It will crank, fire up, then die all within seconds.

Security like cycles through with a few more error messages including low brake fluid, battery, security, and change oil light. It goes through a slideshow of these 4. Hard to tell when it goes off because of the other error messages. Just doing it by timer.

 

It definitely won't start with the PASSLOCK (anti-theft) triggered. It thinks it's being stolen. So focus on that first.

When you unplugged the battery (which wasn't necessary for doing plumbing) , it might have cause it to forget your key or something.

30-Minute Learn Procedure

1. Turn ON the ignition, with the engine OFF.
2. Attempt to start the engine, then release the key to ON; the vehicle will not start.
3. Observe the SECURITY telltale. After approximately 10 minutes, the telltale will turn OFF.
4. Turn OFF the ignition, and wait 5 seconds.
5. Repeat steps 1 through 4 two more times for a total of 3 cycles/30 minutes. The vehicle is now ready to relearn the Passlock™ Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK.
   
   Important: The vehicle learns the Passlock™ Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
   
   
6. Start the engine. The vehicle has now learned the Passlock™ Sensor Data Code and/or password.
7. With a scan tool, clear any DTCs if needed. History DTCs will self clear after 100 ignition cycles.

If that doesn't work then you'll need someone with a dealer level diagnostic tool to scan it.

 

Since it ran good before the radiator replacement, retrace your steps and see if you disconnected something by accident or damaged some wires by accident.