Light when battery is good

It means no tachometer signal seen by the ECM. It makes sense, when you check codes it is usually key on engine off, no rpm's. Another reason, it lets you know the ECM is in diagnostic mode. You should always get the code 12 even when nothing is wrong.

As far as your problem, If you keep the throttle open, just a little, will it run? Usually with idle issues, I would want to check and clean idle air system. How long since a tune-up?

I would probably remove the idle air control valve and see if it needs cleaning. You may need to clean throttle-body and those small passages.

I just wanted to add, if you remove IAC when going back, the pintle should not stick out more than 1 1/8 inches. You can usually work the pintle back with thumb pressure.

Have you checked the fuel pressure when the engine stalls? You might have a fuel pump going out here is a guide to help you get the job done. If you don't have a gauge you can rent one for free at the auto parts store.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

and if the pump is bad here a guide that will give you an idea of what you are in for when doing the job.

https://www.2carpros.com/articles/how-to-replace-an-electric-fuel-pump

Please let us know what you find.

Cheers, Ken