Intermittent Abs And 4wd Warning Lights

[E33]

I am having problems with the ABS and 4WD dash warning lights coming on when started cold.

When the car warms up and then started again most of the time the warning lights go out and all is well for the rest of day or until the car cools down

I can tell prior to starting the car if the warning lights will come on. The ABS motor activates when the ignition is turned on and you can hear it turning. But when it’s cold the ABS motor makes a louder noise prior to engine startup.

This is what I have done

Replaced the ABS unit

Bled the brake lines and ABS as per workshop manual

Checked the ABS computer fault code. Fault 13 was indicated which is no use

Checked the earth strap to the ABS

Anybody had the same fault?

The car is a R32 gtr