P1138 - FAULT CODE - OBD2

POST UPDATED IN January, 2023

P1138 Code failures by brands

P1138 BUICK, CADILLAC, CHEVROLET, GMC - HO2S BANK 1 SENSOR 2 RICH OR HIGH VOLTAGE

Description of DTC code P1138

An HO2S (Oxygen Sensor) is a device whose function is to take readings of the oxygen in the exhaust. Then, it compares these readings to use the data for fuel control and subsequent monitoring of the TWC (Catalyst). The operation of this type of sensor depends on the temperature, which defines its readings. That is why they are equipped with a heater that reduces the time for the sensor to reach the necessary temperature for a good performance.

The Oxygen sensors usually receive 450mV of reference from the ECM (Engine Control Module). When the engine ignition occurs, this module goes into an open circuit and ignores the HO2S voltage. At operating temperature, it goes into a closed circuit and the voltage produced by it changes from 0 to 1,000mV.

Although it is a specific code, the setting of the P1138 OBDII fault code indicates in many cases that the ECM has found a voltage in the circuit of the O2 Sensor number 2 of bank 1, this exceeds the limit established by the manufacturer.

Symptoms of fault code P1138

Causes of OBD2 P1138

The P1138 OBD2 diagnostic code is set due to:

  • Fuel pressure may not be enough.
  • Some vacuum leaks in the motor.
  • Bad wires or connectors in the Oxygen Sensor 1 circuit.
  • A fuel injector could be damaged.
  • The Oxygen sensor 1 is possibly faulty.
  • Contamination levels in the MAF Sensor.

Possible solutions of the DTC code P1138

To troubleshoot DTC P1138 OBD2, try this:

  • First, consult the TSBs.
  • Use a scan tool to check that no other fault codes are stored. If you discover such codes, diagnose them according to the order shown in the tool.
  • Check if the Oxygen Sensor 1 voltage is within the parameters indicated by the manufacturer. You can follow this step with the engine running at operating temperature and using a scanner. If a higher voltage than specified is obtained, it is necessary to verify that the high signal voltage of the sensor on the ECM side matches the one specified. If this value is not as expected, it is necessary to verify that the sensor connector pins are not deficient. Make adjustments if necessary.
  • Check the Oxygen Sensor 1 wiring for defects due to the high temperatures to which it is exposed. Fix if necessary.
  • Check the integrity of the HO2S 1, diagnosing them according to the manufacturer's recommendations. If you discover inconsistencies in part of it, replace it.

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