Recheck the switches. If not already done run truck on blocks remove battery and make sure switches work turning left to right. Operate acc and check for movement in turntable bearing.

I had a similar problem with a fiat, I checked the engines and one was burned, began after I clean the engines, because until then worked with the garbage inside. and failed only to the right.

If the system uses switches for steer lockout, and if its a zapi dc series wound motor controller, then i would expect one of the motors to stop in one lock.

If its not dropping out and the switches and wiring all check out ok, not sure what else to check. I would take a guess and assume the controller should see a feed or have a feed removed by the steer switches which in turn would remove the feed from the contactors (if its a series wound traction motor).

Possible faulty controller??

the fault occurs at the very end of turning to full lock and both drive motors keep driving together instead of one stopping or reversing. I've checked the micro switches under the battery and the wiring,both were ok.

What actually happens?
Does the fault occur whilst on full lock or as turning into full lock and what exactly is the fault?

what message have in the board panel?