P0138 - O2 SENSOR CIRCUIT HIGH VOLTAGE (BANK 1, SENSOR 2)

The Oxygen Sensor 2 (HO2S) is located at the back of the Catalytic Converter (TWC), and its function is to monitor the amount of oxygen in the exhaust gas of each bank. For the Three-Way Catalyst (TWC) can work properly and be most efficient, the air/fuel mixture must be the closest to the stoichiometric ratio. Besides, the oxygen sensor (HO2S) output voltage can change suddenly as it approaches the stoichiometric ratio.

The Engine Control Module (ECM) adapts the time the fuel is injected, so the air/fuel ratio can be either ideal or stoichiometric. The voltage generated by HO2S is 0.1 to 0.9 volts depending on the oxygen in the exhaust. If the oxygen in the exhaust increases, the air/fuel ratio is poor, this happens when the oxygen sensor voltage is less than 0.45 volts. If the oxygen in the exhaust gas diminishes, the air/fuel ratio will be excellent. The Engine Control Module (ECM) interprets excellent when the oxygen sensor voltage is above 0.45 volts.

• The Check Engine service light is lit on the dashboard.
• High fuel pressure is possible, but only in a rich condition.

When the fault code P0138 OBD2 is set, it means that one or more of the following things has happened:

• There may be a short circuit to the battery voltage in the HO2S signal circuit.
• Oxygen sensor 2 (HO2S) may be defective.

To solve an error code P0138 in your car, you can do this:

• You can check the signal circuit of the oxygen sensor, and in case you find broken or worn wires, or even a short circuit, you can make the necessary repairs.
• You could replace the oxygen sensor (HO2S).