The oxygen sensor monitors the fuel-to-air ratio in the spent exhaust near or on the manifold. Since the integration of OBD II (on-board diagnostics) in 1996, the amount of oxygen sensors on vehicles has doubled. Upstream sensors still monitor the exhaust from the manifold; downstream sensors now monitor the catalyst efficiency near the catalytic converters. While replacing some oxygen sensors is a simple task, vehicle designs can frustrate the most seasoned mechanics when it comes to replacing them due to lack of access. Tool companies advance along with technology. Luckily, there's always the right tool for the job.
Most oxygen sensors screw into portholes or bungs in the exhaust system. Due to extreme heat from the exhaust of the combustion engine, the pipes of the exhaust system heat up when operating and then cool down after the vehicle is shut off. This creates an unfriendly environment for the metals of the exhaust pipe and the threads of the oxygen sensor. Even sensors that are easily accessible can provide a high degree of difficulty when trying to be unscrewed from the exhaust pipe.
Rest assured that there is the right tool to remove the most challengingly accessible oxygen sensor. While many offer easy access, today's tools will offer you a variety of choices as well as combining other common tools in order to successfully remove oxygen sensors.
Oxygen sensor wrench with flex heads are now available. Oxygen sensor sockets with slotted edges are also quite popular. Wrenches that fit onto the hex-head of the sensor--most commonly a 22-mm or a 7/8-inch drive--accompanied with a square drive in the wrench to accommodate a ratchet extension can be used. Swivels are another friendly tool for access-challenged sensors. And in the easiest ones to remove, a box-end wrench can even be used.
Always unplug the wire harness of the sensor before attempting to remove it.
So now, you've found the right tool or the right combination, you've unplugged the wire and placed it through the wrench or into the slot of the socket and the sensor is still stuck. While some suggest heating up the engine by running it will help, the fact is, under this procedure, the heat expands both the exhaust pipe and the sensor threads. Penetrating lubricant in liberal doses may help in this manner, but if you have a small propane or oxyacetylene torch, you can also heat the edges of the bung on the exhaust pipe. This will expand the pipe and bung without greatly affecting the sensor. A little heat will go a long way when removing stubborn sensors.
It's also a good idea to allow the pipe to cool down and then chase or tap the bung with an O2 sensor thread tap or chaser. Commonly an 18-mm diameter with a 1.5-mm thread pitch is used, but check the sensor threads and diameter at the parts store when you buy it to make sure. This will make threading the new sensor easier. Since the threads of oxygen sensors are much weaker than the bung, you can easily damage or cross-thread a new sensor trying to install it without chasing the threads of the bung. Apply a little anti-seize lubricant on the threads of the sensor--being careful not to get any on the head of the sensor--before tightening; although many quality aftermarket sensors will provide some anti-seize compound on the threads for you already.
