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To identify where instability comes from, you isolate the controller from the loop and observe the plant by putting the controller into manual mode. When in manual, the controller stops making automatic adjustments to the manipulated variable, so you can see how the process responds on its own to a fixed input. If the process variable still behaves irregularly or oscillates, the problem is in the plant, sensors, delays, or other parts of the process—not the controller. If the process settles or becomes stable with the controller in manual, the instability was being caused by the controller’s settings (gain, dynamics, filters) and would likely be resolved by retuning. Other options either introduce more instability, remove feedback, or add delay, which don’t clearly pinpoint the source in the same way.