P0138 Code - O2 Sensor Circuit High Voltage Bank 1 Sensor 2

What Does P0138 Mean?

The internal combustion engine ignites a hydrocarbon (HC) fuel, combining it with oxygen (O2), using the resulting energy output to turn the engine. The exhaust stream, ideally, is composed simply of water (H2O), carbon dioxide (CO2), and unreacted nitrogen (N2). Unfortunately, due to inconsistencies in engine operation, air and fuel temperature, fuel composition, and a number of other factors, exhaust chemistry can be far from ideal. Excessive cylinder temperatures, for example, can lead to the production of nitrogen oxides (NO and NO2). An excessively-rich air-fuel-ratio (AFR) can lead to elevated levels of unburned HC. Other conditions can lead to the production of carbon monoxide (CO), ozone (O3), and PM10 and PM2.5 (particulate matter less than 10 µm or 2.5 µm in size).



To reduce these combustion emissions, the engine control module (ECM) monitors air and fuel temperature, intake air mass and flow, combustion pressure, engine speed and load, and more to fine-tune AFR, valve timing and lift, spark timing, and others. Still, even these controls are not enough to prevent some harmful emissions from forming, so the last step is the three-way catalytic converter (TWC or “Cat”), a chemical filter of a sort, that recombines harmful emissions into safer compounds. Unburned HC and PM2.5 and PM10 emissions combine with CO, O3, O2 to produce H2O and CO2. The ECM compares oxygen sensor readings before and after the TWC to monitor its performance.

The AFR sensor, before the TWC, is used mainly as feedback to fine-tune fuel trim (FT), but also is used by the ECM to test the TWC. Under typical operation, the ECM increased and decreases FT, always attempting to zero in on 14.7:1 AFR, using the AFR sensors to verify. Normal AFR sensor readings typically fluctuate high/low a few times a second. If the TWC is functioning properly, the heated oxygen sensors (HO2S) will barely fluctuate at all. Every few seconds, the ECM will force FT higher or lower than normal, which the AFR sensors will react to by showing higher or lower voltage than normal. If the TWC is functioning properly, the downstream HO2S should barely react. If the HO2S voltage peaks at the same time as the AFR sensor voltage, this ECM determines that the TWC is not functioning properly and sets a DTC (diagnostic trouble code) related to TWC function.



Before the ECM can monitor the TWC, however, it needs to confirm that the AFR and HO2S are functioning properly, so it checks the heater and signal circuits for proper voltage and resistance. If the ECM detects signal voltage higher or lower than possible, it will illuminate the malfunction indicator lamp (MIL) and store a DTC in system memory, defined as follows:

• DTC P0138 Heated Oxygen Sensor (HO2S) Circuit High Voltage Bank 1 Sensor 2 (B1S2)
• For reference, Bank 1 and Bank 2 refer to which bank of cylinders, left or right if it’s a V configuration, Bank 1 containing Cylinder 1. Inline engines may be divided, Bank 1 being Cylinder 1 & 2 or 1 – 3. Sensor 1 or Sensor 2 refers to the sensor’s position before or after the TWC. Sensor 1 is before the TWC and Sensor 2 is after the TWC.

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What are the common causes of P0138 ?

• An air leak forward of the DPF will change the sensor readings resulting in the code
• Rusty exhaust systems, cracked exhaust manifolds or damaged or missing gaskets or donuts will cause air leaks.
• A large air leak at or near the number one oxygen sensor would also cause a lean mixture.

What are the symptoms of P0138 ?

• Check Engine Light will illuminate
• Vehicle may idle or run rough
• Decrease in fuel economy
• Bad smelling exhaust
• In some unusual cases, there are no adverse conditions noticed by the driver

How do you troubleshoot P0138 ?

• Replacing the O2 sensor for bank 1 sensor 2
• Repairing or replacing the wiring or connection to the O2 sensor for bank 1 sensor 2
• Replacing the catalyst in front of the sensor
• Repairing a leaking injector

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