I had few occasions of "re-repair" after someone's intervention because of "Motor locked - Mod 06" error on Yale MR14-20 models,the last one was few days ago...
When this error appears, the truck can be turned on but immediately show the error and can not be used.
The first thing you will notice in this occasion (if you are familiar with this model) there is no immediate steering wheel centering (there is not at all) which is a standard function of truck after switching on though there are errors or not.

Repair manuals says:
ALARM: 06 MOTOR LOCKED
COMPONENT: Steering motor encoder feedback
ACTIONS REQUESTED: Check encoder connections and renew encoder if necessary

When you open the hood you will see that the steering motor really does not work,you will disassemble it from the chassis and even then if you try to turn on the truck motor will not work and that is (usually) a trap.

The steering gear/reduction gear is along steering motor and check of gear is often overlooked by technicians and usually there is the problem.
Motor and gear must be disassembled when you want to change the encoder on steering motor and interesting thing about steering gear is that it will work ( not always) for a while after disassemble/assemble procedure on the motor but not for long,but it's seems the problem is solved...
Advice is to disassemble steering motor and gear on the spot (it's really simple) and turn on the truck.In most cases motor will spin and you know the problem is in the gearbox,not in motor's encoder.
This case is very common on truck with over 5000 working hours.
My practice is to dissasemble and grease gear box on every 2000 hours, if you do,pay attention on gears inside, because it is planetary reducton gear.
You will save customer money, your time and your reputation.