Negative fuel trim at idle can be caused by a few different issues. It could indicate a vacuum leak, faulty oxygen sensor, fuel pressure regulator problem, or even a problem with the mass airflow sensor. These issues can lead to an imbalance in the air-fuel mixture, causing the engine to run too rich. It's important to have a qualified mechanic diagnose and address the root cause of the negative fuel trim to prevent further damage to the engine and ensure optimal performance.