Fault Phenomenon: Caterpillar AP555F paver when auto-leveling is active, the screed slowly drifts down or up on its own.

Troubleshooting Process:
– Checked spool: proportional valve spool worn, internal leakage.
– Checked electrical: control current zero-point drift.
– Checked hydraulic lock: cylinder hydraulic lock internal leakage.
– Checked sensor: tilt sensor zero-point drift.

Root Cause: The spool of the leveling control proportional solenoid valve is worn, allowing a small amount of oil to pass even in the neutral position, causing the cylinder to move slowly.

Required Parts: Replace leveling proportional solenoid valve assembly.