Do you have a manual? The guy I'm buying it off can't find the serial number for the unit.Welcome to HEF dooner .
Hitachi ZX135UR. From the troubleshooting manual TT187E-04 .
“Code UE81, Arm angle sensor deviation.
Check arm angle sensor installation.”
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I assume there is a position limit system on the machine and there is a sensor at the arm to boom joint.