On the 2003 Toyota Matrix, the P0136 is a very straight forward code pointing to a problem with the “downstream” (bank-1/sensor-2) oxygen sensor. These are very common wear-and-tear fail items. The sensor is located under the vehicle, mounted to the rear of the catalytic converter. Carefully inspect the electrical connector and wire harness for any loose, broken or damaged wires. If no problems are found, replacing the sensor should solve the problem. To replace, unplug the electrical connector and unscrew the sensor from the exhaust.
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