The P0138 code signals a high voltage reading from the downstream oxygen sensor on Bank 1 (sensor 2). In many cases, the fix is to inspect and, if necessary, replace the affected sensor or its wiring. This article outlines what the code means, how to diagnose it, and practical repair steps you can take.
Bank 1 Sensor 2 is the oxygen sensor located after the catalytic converter on the side of the engine that contains cylinder 1. The downstream sensor monitors catalytic converter efficiency and helps the engine control system fine-tune emissions and fuel trim. While leaks and exhaust issues can influence readings, the most common causes are a failing sensor, damaged wiring, or a poor connector. The goal is to confirm the fault, isolate the cause, replace components as needed, and re-test to ensure the code does not return.
What P0138 Means
P0138 is a diagnostic trouble code that indicates the O2 sensor in Bank 1 Sensor 2 is reporting an abnormally high voltage. This usually points to a problem in the sensor circuit, the sensor itself, or its wiring. Because this sensor is downstream of the catalytic converter, it is primarily used to monitor converter performance rather than directly controlling engine air-fuel mix. A high reading can trigger a check engine light and may affect emissions data, so timely diagnosis is important.
Bank and Sensor Identification
In most vehicles, Bank 1 Sensor 2 is the oxygen sensor located after the catalytic converter on the side of the engine that houses cylinder 1. It reads the exhaust after it passes through the catalytic converter and informs the engine control module about catalytic efficiency.
Common Causes
Several issues commonly lead to P0138. Identifying the exact cause requires a systematic check of the sensor, wiring, and exhaust conditions.
- Faulty downstream O2 sensor (Bank 1 Sensor 2)
- Damaged or corroded wiring or connectors in the sensor circuit
- Electrical shorts to power or ground in the sensor circuit
- Exhaust leaks near the sensor or issues affecting post-cat readings
- Sensor contamination or wear affecting sensor output
- Rare cases of a faulty PCM/ECU
Understanding these causes helps focus diagnostic efforts on wiring integrity, sensor health, and exhaust integrity before considering more involved repairs.
How to Diagnose P0138
Follow a structured approach to confirm the fault and locate the source. The steps below are practical for a DIY check and to guide what a shop might do.
- Use a capable OBD-II scanner to confirm the code and view live data for Bank 1 Sensor 2 voltage and heater status.
- Check freeze frame data to understand engine conditions when the code was stored.
- Inspect the Bank 1 Sensor 2 location for physical damage, corrosion, or signs of exhaust leaks around the sensor bung.
- Inspect the wiring harness and connectors to Bank 1 Sensor 2 for fraying, broken pins, or loose connections.
- Test the sensor circuit with a multimeter: check that the signal wire shows voltage in the expected range and that there are no shorts to power or ground.
- Test the sensor’s heater circuit (if equipped) for proper resistance and operation according to the vehicle’s service data.
- Consider a sensor swap test: temporarily swap the Bank 1 Sensor 2 with a known-good sensor or use a cross-reference if feasible to see if the fault follows the sensor.
- If the fault persists after wiring and sensor checks, proceed with replacing Bank 1 Sensor 2 O2 sensor and clear codes to re-test.
After these steps, perform a road test under normal driving conditions to verify that the code does not return. If it does, further diagnosis of the exhaust system or ECU may be required.
Repair Options
Repair choices depend on what the diagnostic steps reveal. The most common remedies are sensor-related, but wiring and exhaust integrity are equally important to rule out.
- Replace Bank 1 Sensor 2 O2 sensor if it is faulty or not reporting correctly.
- Repair or replace damaged wiring or connectors to the Bank 1 Sensor 2 circuit; fix grounding issues or damaged power feeds.
- Repair exhaust leaks around the downstream sensor or post-cat area, which can cause abnormal readings.
- Address underlying engine issues (misfire, lean/rich conditions) that could affect downstream sensor readings.
- Re-test after repairs, clear DTCs, and perform a road test to confirm the code does not return.
Note that not all P0138 cases require a sensor replacement; wiring fixes or leak repairs can resolve many instances. If in doubt, consult a professional technician for diagnostic testing with appropriate equipment.
When to Seek Professional Help
If you cannot locate or fix the issue with the steps above, or if the vehicle fails emissions testing, seek a certified mechanic. Persistent O2 sensor faults can impact fuel economy and exhaust emissions.
Summary
P0138 signals a high-voltage reading from Bank 1 Sensor 2. The pragmatic path to resolution is to check the sensor, inspect wiring and connectors, and address exhaust leaks or underlying engine issues. If needed, replace the sensor and re-test to ensure the code does not return. A thorough road test confirms the repair’s effectiveness.
How long can I drive with a P0138 code?
Can I safely drive my car with this error code? Yes, you can typically drive with the P0138 code, but it's advisable to get it repaired soon. Driving with a rich-running engine can lead to higher fuel costs, increased emissions, and potential long-term damage to the catalytic converter, which is expensive to replace.
Where is the oxygen sensor on a P0138?
The P0138 error code can be defined as O2 sensor circuit high voltage bank 1, sensor 2. It is normally located in the catalytic converter, before the muffler, in the car exhaust system. The oxygen levels are monitored in this sensor in the exhaust stream.
How much does it cost to replace the oxygen sensor on a Chrysler 300?
The average cost for a Chrysler 300 Oxygen Sensor Replacement is between $254 and $342. Labor costs are estimated between $58 and $86 while parts are priced between $196 and $257.
What is a common cause of P0138?
The most common cause of the P0138 code is a faulty O2 sensor. Over time, these sensors can gert contaminated or burnt out, leading to incorrect voltage readings. If the sensor is damaged or badly coated with carbon, it can't accurately monitor the oxygen levels in the exhaust gases, triggering the P0138 code.