[Mickey#1]

Cables & grounds should be the 1st thing checked - it doesn't cost anything.
If your Regal has 2 battery cables that come together under a single rubber boot then you have to get the boot off to clean the cable ends.

I'd also remove the coils & ICM & check for corrosion between the mounting plate & ICM. Another freebie.

While the coils are off you can check the resistance of the primary & secondary windings. A weak coil won't throw a code.

When was the last time the plugs & wires were changed? I'd check them before getting an ECM.