Fault Phenomenon: Dingli GTBZ40 telescopic boom aerial work platform boom movement is uncontrolled, speed is erratic or stuck.

Troubleshooting Process:
– Checked spool: spool stuck by contaminants.
– Checked coil: unstable coil signal.
– Checked fluid quality: fluid dirty.
– Checked filter: filter failed.

Root Cause: Dirty hydraulic fluid caused the proportional valve spool to stick.

Required Parts: Proportional valve assembly and hydraulic oil change.