Ender 3 v3 KE, automatic height anomoly, Z-axis too high

brand new printer. worked for a few prints initially. then it started setting lowest Z too high. manual drive of Z says it hits the limit probably 5 mm from bed.
upgrading firmware didn’t fix it.
factory reset calibration fails with error 4000 and an “automatic height anomaly” message.
chainging the z offset does’t lower it enough.
where is the z-axis sensor?

The CR touch is the Z sensor, is it lit up purple? I would check its connector is seated and probably reseat the ribbon cable connection.

correction: it’s error key: 2401.

tried your suggestion. i reseated the main ribbon cable. I didn’t touch the sensor’s cable.
It prints the first line on initial calibration, then does a couple more height tests and fails when the pin does not drop and z goes up and down quickly. same fail pattern every time.

the light is purple when not testing.
blue when up.
red when hitting bed.

firmware: V1.1.0.12

Back off the grub screw that acts against the magnet? It may be too close for the probe to redeploy?

What grub screw near the CR touch? I didn’t see one. it’s an enclosed mini module, mounted with two screws.