How do you troubleshoot a control loop that is oscillating?

Systematic approach: verify oscillation source (not an external periodic disturbance), check for valve issues (stiction, sizing, saturation, hysteresis), review recent tuning changes, check for interaction with other loops. If controller-induced: reduce proportional gain, increase integral time, reduce derivative (or eliminate). Check: valve position cycling, control output vs PV relationship. Use trend analysis - sustained oscillation with constant amplitude suggests limit cycle (often valve stiction); decaying oscillation suggests aggressive tuning. Check valve signature for stiction diagnosis.