depending on which steering controller this truck has:
EPSIII
you may also want to check the encoder spherical connector that connects the pot to the steer gear shaft. I've seen those break and cause errors like this. The set screws can come loose or the coil in the connector can break. Ultimately the encoder may just be faulty.

APS steering
could be a bad position sensor

Steer encoder error channel count mismatch


Possible causes...

Check wiring to encoder.
Check both encoder output channels using voltmeter; voltmeter should pulse with encoder rotation
. Check for chassis ground in contact with encoder housing.
Check for wiring problems on external 1243 capacitor.