This week I went back to the customer to check on this WAVE. No problem they say. Checked for errors; no error. Conclusion: controller fault.

Today I have taken out the NEO motors and blow them with compressed air. There was some brush dust. The brushes are 12 mm, which is OK. Commutator - clean copper. :-)

I have Curtis handset. When operating the rocker from side to side it shows 100% right & 82% left. I have calibrate it a few times. Well, I think I did. I never receive an confirmation from the Curtis handset.

Once, while turning right, WAVE stopped and the handset was showing 100% right from potentiometer.

I've changed the complete left handle again and controller also from another WAVE. The customer will run it for a few days and will see...

Good idea about the castor wheels. I will check that also.

Wave drive motors are notorious for problems. I'm not familiar with the the new style neo motors, but the earlier motors were often never cleaned out by the techs, and the brushes were usually not replaced until there was a motor fault code, which by that time it was usually to late and motor damage was done.

That being said, steer throttle fault is a control system problem, not drive system. Do you have the Curtis handset to plug into this machine for diagnostics? If not, you need one, it's very difficult to do good diagnostics on the wave without it. With it you will actually be able to see what's going on with the pots and controller. I'm not sure that you can properly calibrate the pots with out it either.

Just a side thought, have you checked that the front wheels are turning freely?

No offence taken. People hire me. :-)

This WAVE has new type NEO motors installed with only a few work hours. I've checked the brushes few weeks ago during service but I will check them again. I've also checked the part of wiring under the platform and it seem alright.

"Steer throttle" error concerns steering potentiometer and it's wiring. This complete circle is inside left handle. But when I change the complete handle from another WAVE the problem remains.

Any other ideas?

All the electrical system is isolated from the truck chassis.
As carbon brushes wear down the dust collects inside the motor and over time begins conducting to the chassis causing all sorts of weird things. The wave has dual drive motors and if the one for turning left is "dirty" it could cause this.
If you are not familiar with forklifts it might be best to hire someone. No offence, I have tried assisting people with no repair background before with limited success.

What do you mean? Example?

Chassis shorts would be a good start.
Motors can ground out.