If you have a P0138 code, it means that the oxygen sensor in your vehicle is not functioning correctly. The oxygen sensor is responsible for monitoring the level of oxygen in the exhaust gases. If the sensor is not working properly, it can cause the engine to run lean or rich. A P0138 code can be caused by a number of things, including a faulty oxygen sensor, a clogged catalytic converter, or an exhaust leak.
To fix a P0138 code, you will need to replace the oxygen sensor. You may also need to replace the catalytic converter or repair an exhaust leak.