How do you diagnose and fix oscillating control loops?

Oscillation diagnosis: determine oscillation type - continuous sustained (controller at stability limit), decaying (acceptable if reasonable), increasing (unstable, immediate action needed), or irregular (external disturbance, valve sticking). Diagnostic steps: put controller in manual to check if oscillation stops (controller-induced vs external), check for valve stiction/hysteresis (monitor valve position vs output), examine interaction with other loops, and verify sensor signal quality. Fixes: reduce controller gain, increase derivative for slow oscillation, fix mechanical issues (valve, actuator), or retune using appropriate method.