2001 Alero won't start. It died on the road and hasn't started. I put the diagnostics test but it showed no errors. What can it be?

Have you checked the PCL in the car? Like the computer of the car? You might need to tow it to an autozone or somewhere similar and have it checked.

Question edited for clarity and typos.
Question moved to model category.

It is a known issue. Fuel pumps can work, but not provide enough pressure, so you need to test with a pressure gauge. There is also a Pressure Regulator. Have a read of this.

https://www.justanswer.com/oldsmobile/6cz1n-oldsmobile-alero-olds-alero-starts-dies.html

Lots more here

https://www.google.com/search?q=2003+Oldsmobile+Alero+starts+and+then+dies

You may need an auto electrician with a pro grade scan tool computer.

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You have to find out what stopped working , ignition system , fuel pump , engine malfunction etc.... Videos on youtube showing how to test these thing's . No Start Diagnostic Basics EricTheCarGuy

Mass air flow , crank sensor . I wouldn't discount anything . Having a scan tool that can view engine sensor data would be a great help too , testing fuel pressure and flow and back pressure test the exhaust .
058 Engine Hard to Start or No Start Lean Fuel Condition
055 Engine Hard to Start or No Start Scan Data for EGR
057 Engine Hard to Start or No Start Ignition Module Test
007 Catalytic Converter Testing

Look for a light that says SECURITY or ANTITHEFT on the dash. This light could cause this problem. Explained on this link.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

probably made a harness connection to the fuel pump come apart in the fitting the new transmission

If it does, a good way to test it would be take off the positive (+) side of your battery. Wait a few minutes, then put it back on. Hope this helps!

change the starter motor.

check diagnostic codes.could possibly be crankshaft and camshaft position sensor

try this site:
http://www.obd-codes.com/p0420