P0138 Code on a Corolla: Causes and Solutions

P0138 Code on a Corolla: Causes and Solutions

Quite a few Toyota owners are experiencing a P0138 code on their Corolla. A P0138 extremely high voltage (short) is set when the oxygen sensor reports voltage above 1.2v for 10-secs or more on bank 1 sensor 2. Sensor 2 is located after the catalytic converter and should normally produce a near flat line response.

The car’s computer conducts tests to ensure the oxygen sensors and catalytic converters are operating properly. This Toyota test is called active air fuel ratio control. The computer forces a rich or lean mixture into the engine and then reads the results from the oxygen sensors to see if they respond in a predicted manner. For example, if the oxygen sensor doesn’t increase to 0.59b during the test, the computer will set a P0136 code. If it’s above 1.2v, it sets a P0138.

How to test oxygen sensor for P0138 code on a Corolla

Disconnect the electrical connector to the post catalytic converter air/fuel (oxygen) sensor. The connector has 4 wires. Terminal 1 (pink) is fused battery voltage for the sensor heater and terminal 2 is the ground for the heater. Terminal 4 (yellow) is a 5-volt reference from the ECM and terminal 3 (red) is the actual O2 signal to the ECM.

Use this wiring diagram to troubleshoot a P0138 code on a Corolla

With the key in the RUN position, check for battery voltage on terminal 1 and 5-volt reference on terminal 4. Check for good ground on terminal 2.

If the voltages and ground check out, look for corrosion in the connector due to water dripping on the connector from the AC condensate drain. Also check for chaffing on the wiring harness. Both are well known problems that can result in a Corolla P0138 trouble code.

©, 2019 Rick Muscoplat