O
2 sensors seal with a gasket similar to those used on 18-mm flat-seat spark plugs. They can be difficult to break loose if they have been in service for a long time. Occasionally, they become so thoroughly seated that they can actually pull the threads right out of a cast iron manifold during removal. Generally this occurs when the sensor was either over tightened, installed into a dirty or cross threaded hole, or reused without the appropriate thread preparation. Sensor removal is ofttimes easier when the manifold is warm, but not too hot to touch with a bare hand. Do not apply penetrating oil unless you plan on discarding the sensor.
Use a 7/8" box wrench or a special 1/2"-drive O
2 sensor socket for removal. However, be advised that many of the special sockets have a machined slot on one side to accommodate the sensor lead. Although the special sockets are thick walled, the slot can weaken the socket and make it susceptible to spreading. Hence, if the sensor is really tight, a box wrench is the better choice; but cushion your hand using a rag or thick glove. If the old sensor is difficult to unscrew, chase the threads in the bung before installing the replacement.
The threads on OE sensors are coated with a glass bead anti-seize lubricant intended for one time insertion. Because of high exhaust temperature and the material used for the shell, if you must reinstall a used sensor or you are installing an uncoated aftermarket sensor, lightly coat the sensor threads with
nickel anti-seize. But, keep the louvered end of the sensor totally free of any lubricant or foreign substance. Always install an O
2 sensor into a
cold manifold or bung. Sensors must be snug (similar to tightening a spark plug), but be careful to avoid overtorquing.
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