The exposed portion of the oxygen sensor, as well as the tip and signal wires, are subject to damage and should be inspected whenever work is performed. Check the following items carefully:
(1) Signal Wire for cracks or damage

(2) Air intake blocked by dirt, oil or undercoating

(3) Sensor Body for any damage

(4) Threads for wear or cross threading

(5) Sensor Tip for damage or contamination.

The tip of an oxygen sensor (5) must be clean in order to function properly. A contaminated tip cannot detect the oxygen in the exhaust properly and may give false rich condition readings. Typical contaminants are:
Carbon Buildup (Blackened Tip),

Lead Contamination (Glossy Deposits)

Silicone Contamination (White Chalky)

Oil Contamination (Dark Brown),

Antifreeze Contamination (White Grainy)

Replace any malfunctioning oxygen sensor immediately to prevent additional damage to the exhaust system components.