code C1 is 12v power supply failure

reason is 12v is not being sensed at the steer controller

system response is:
horn sounds
display shows C1 (check battery)
speed limit: 0.0mph (dump brake occurs)
code on steer controller card appears as C 1

if you look under hood on tractor unit you will find microcontroller on right hand side, the steering controller will be mounted about center of the truck on same shelf microcontroller is. It will have cables and several harness connectors going to main harness and down to drive motor and steer motor area, they will be connected to the steer motor and one cable will be connected to microcontroller.
the display on the steer controller is 2 LED windows and both will show a C1, you may also see a C1 on the microcontroller LED display.

check on the steer controller to see if the 12v LED on the controller is on.

if code is intermittant you may need to run maintenance test A50 to check steer controller supply voltage test.

also first and foremost check the battery voltage, run a load test to see if battery is holding up well, if there is severe voltage drop then battery may be causing the problem.