P0138 – O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Fault Code Description
The P0138 Diagnostic Trouble Code (DTC) is a generic OBD-II powertrain fault code that indicates the Engine Control Module (ECM) has detected a continuously high voltage signal from the Bank 1 Sensor 2 oxygen sensor. This sensor is located downstream of the catalytic converter and is primarily used to monitor catalytic converter efficiency rather than control the air-fuel mixture.
Under normal operating conditions, the downstream oxygen sensor voltage changes slowly, typically ranging between approximately 0.1V and 0.9V. If the ECM detects that the sensor voltage remains excessively high (typically above about 0.9V) for longer than the manufacturer's specified time, it stores the P0138 code and may illuminate the Check Engine Light (MIL).
Although the downstream oxygen sensor has little influence on fuel delivery in most vehicles, a persistent high-voltage signal may indicate a failed oxygen sensor, damaged wiring, a short to voltage, excessive fuel richness, or, in rare cases, an ECM fault.
What Does Bank 1 Sensor 2 Mean?
- Bank 1 — The side of the engine containing cylinder number 1.
- Sensor 2 — The oxygen sensor located after the catalytic converter.
Because the sensor is installed after the catalytic converter, its primary function is to verify converter efficiency by comparing exhaust oxygen levels before and after the catalyst.
Code Summary
P0138
O2 Sensor Circuit High Voltage
Powertrain
Bank 1 Sensor 2
After Catalytic Converter
Moderate
Bank 1 Sensor 2 Location and Function
The Bank 1 Sensor 2 (B1S2) oxygen sensor is installed after the catalytic converter on the exhaust system. Unlike the upstream oxygen sensor, this sensor is not primarily responsible for controlling the air-fuel ratio. Instead, it monitors the catalytic converter's efficiency by measuring the oxygen content in the exhaust gases after they pass through the converter.
The Engine Control Module (ECM) compares the signal from the downstream oxygen sensor with the upstream sensor. If both sensors produce nearly identical voltage patterns, the catalytic converter may not be storing oxygen efficiently. However, when the downstream sensor voltage remains excessively high for an extended period, the ECM may store the P0138 trouble code.
Typical Sensor Location
Typical Oxygen Sensor Pinout
Heater Power (B+)
Heater Ground
Sensor Signal
Sensor Ground
Note: Wire colors and terminal assignments vary between manufacturers. Always consult the factory wiring diagram for the specific vehicle before testing.
Symptoms of Code P0138
The symptoms associated with the P0138 trouble code vary depending on the underlying cause. In many vehicles, the driver may notice only the illuminated Check Engine Light because the downstream oxygen sensor is mainly used to monitor catalytic converter performance. However, if the high-voltage signal is caused by an excessively rich fuel mixture or an electrical fault, additional drivability issues may occur.
🚨 Check Engine Light
The Malfunction Indicator Lamp (MIL) is usually the first and most common symptom.
⛽ Increased Fuel Consumption
If the engine is running excessively rich, fuel economy may decrease noticeably.
🚗 Rough Engine Operation
Some vehicles may experience rough idle or hesitation if the rich condition is severe.
💨 Black Exhaust Smoke
An overly rich air-fuel mixture may produce dark exhaust smoke.
🔥 Fuel Smell
A strong gasoline odor may be present near the exhaust or around the vehicle.
⚠️ Failed Emissions Test
High exhaust emissions may cause the vehicle to fail an emissions inspection.
Can You Drive with Code P0138?
In most cases, the vehicle can still be driven for a short period. However, prolonged operation with a rich fuel mixture can damage the catalytic converter, increase fuel consumption, and lead to additional engine performance problems. The fault should be diagnosed and repaired as soon as possible.
Severity
Immediate engine damage is uncommon, but ignoring the P0138 code may result in catalytic converter damage, higher fuel costs, and increased exhaust emissions.
Causes of Code P0138
The P0138 trouble code is stored when the Engine Control Module (ECM) detects that the Bank 1 Sensor 2 oxygen sensor signal remains excessively high for longer than the calibrated time. This condition may be caused by a faulty oxygen sensor, damaged wiring, or an engine operating with an excessively rich air-fuel mixture.
1. Faulty Oxygen Sensor
The most common cause is an internally failed downstream oxygen sensor that becomes stuck at a high voltage output.
2. Short to Battery Voltage
A damaged signal wire touching a battery voltage source can force the sensor signal to remain high continuously.
3. Rich Air-Fuel Mixture
Excess fuel entering the engine may increase exhaust oxygen sensor voltage. Possible causes include leaking fuel injectors, excessive fuel pressure, or incorrect fuel control.
4. Damaged Wiring or Connector
Corroded terminals, broken insulation, melted wiring near the exhaust, or water intrusion may create abnormal sensor readings.
5. Poor Sensor Ground
A weak or unstable sensor ground can alter the reference voltage and produce an artificially high sensor signal.
6. ECM Failure (Rare)
Although uncommon, an internal fault in the Engine Control Module may incorrectly interpret or generate the oxygen sensor signal.
Common Causes Ranking
| Possible Cause | Likelihood |
|---|---|
| Failed Bank 1 Sensor 2 Oxygen Sensor | ★★★★★ Very Common |
| Short to Voltage | ★★★★☆ Common |
| Rich Fuel Mixture | ★★★★☆ Common |
| Connector or Wiring Damage | ★★★☆☆ Moderate |
| Poor Ground Circuit | ★★☆☆☆ Occasional |
| ECM Failure | ★☆☆☆☆ Rare |
Important Note
A high oxygen sensor voltage does not always mean the oxygen sensor is defective. Before replacing the sensor, inspect the wiring, verify the sensor ground, and check whether the engine is operating with an excessively rich air-fuel mixture. Replacing the sensor without proper diagnosis may not resolve the problem.
How to Diagnose Code P0138
A systematic diagnosis is essential before replacing the oxygen sensor. Although a faulty Bank 1 Sensor 2 oxygen sensor is a common cause, wiring faults, a rich air-fuel mixture, or poor electrical connections can produce the same DTC.
Step-by-Step Diagnostic Procedure
Step 1
Confirm the DTCConnect an OBD-II scan tool and verify that P0138 is active or stored. Record Freeze Frame Data before clearing any codes.
Step 2
Inspect the Oxygen SensorCheck Bank 1 Sensor 2 for physical damage, contamination, or signs of overheating. Verify the connector is fully seated.
Step 3
Inspect the Wiring HarnessLook for melted insulation, damaged wires, corrosion, loose terminals, or a short-to-voltage condition caused by contact with the exhaust system.
Step 4
Check Live DataObserve the Bank 1 Sensor 2 voltage using a scan tool. A voltage stuck near 0.9–1.0V continuously indicates an abnormal condition requiring further testing.
Step 5
Verify Engine Fuel ControlInspect Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT). A rich-running engine may be responsible for the high sensor voltage.
Step 6
Measure the CircuitUsing a digital multimeter, verify heater power supply, heater ground, sensor ground, and signal circuit continuity according to the manufacturer's specifications.
Step 7
Replace the Sensor if NecessaryIf wiring and engine operation are normal but the sensor output remains fixed at high voltage, replace the downstream oxygen sensor.
Recommended Diagnostic Order
Diagnostic Procedure
Follow the steps below in order. Do not replace the oxygen sensor until the wiring, power supply, ground circuit, and live data have been verified.
Step 1 – Read DTCs
Connect an OBD-II scan tool and record all stored and pending trouble codes. Freeze Frame data can help identify the conditions under which P0138 was detected.
Step 2 – Inspect Wiring
Check the Bank 1 Sensor 2 harness for melted insulation, broken wires, corrosion, loose terminals, or contact with the hot exhaust system.
Step 3 – Monitor Live Data
Observe the Bank 1 Sensor 2 voltage using a scan tool. A sensor stuck near 0.9–1.0V continuously is abnormal and requires further testing.
Step 4 – Test the Heater Circuit
Verify battery voltage at the heater power wire and check the heater resistance according to the manufacturer's specifications.
Step 5 – Check for Rich Operation
Inspect fuel trims, leaking injectors, excessive fuel pressure, and other conditions that may produce a continuously rich exhaust mixture.
Step 6 – Replace Only After Testing
If wiring, power, ground, and engine operation are normal, replace the downstream oxygen sensor and clear the fault code.
Note: These diagrams represent typical signal behavior only. Actual wiring, pin assignments, and voltage characteristics vary by manufacturer and engine management system.
Repair Methods for Code P0138
After completing the diagnosis, repair the root cause before clearing the Diagnostic Trouble Code (DTC). Simply replacing the oxygen sensor without confirming the actual fault may not solve the problem.
1. Replace the Oxygen Sensor
If Bank 1 Sensor 2 is confirmed to be faulty or its output remains fixed at high voltage after testing, replace it with a high-quality OEM or equivalent sensor.
2. Repair Damaged Wiring
Repair or replace melted, broken, or shorted wires. Ensure the wiring harness is routed away from the exhaust system and secured properly.
3. Clean or Replace the Connector
Remove corrosion, repair loose terminals, and replace damaged electrical connectors if necessary.
4. Correct Rich Fuel Mixture
Repair leaking fuel injectors, incorrect fuel pressure, or other engine management faults that cause an excessively rich air-fuel mixture.
5. Repair Heater Circuit
Restore proper heater power and ground if the oxygen sensor heater circuit is faulty.
6. ECM Repair (Rare)
If all sensor and circuit tests pass, further diagnosis of the Engine Control Module (ECM) may be required.
Estimated Repair Cost
| Priority | Inspection Item | Recommended Tool |
|---|---|---|
| 1 | Read DTC & Freeze Frame | OBD-II Scanner |
| 2 | Visual Wiring Inspection | Flashlight |
| Repair | Estimated Cost (USD) |
|---|---|
| Replace Oxygen Sensor | $80 – $350 |
| Repair Wiring | $50 – $250 |
| Replace Connector | $30 – $120 |
| Fuel System Repair | $150 – $900+ |
| ECM Diagnosis/Replacement | $300 – $1,500+ |
Common Repair Mistakes
- Replacing the oxygen sensor without checking the wiring.
- Ignoring a rich-running engine that is causing the high sensor voltage.
- Failing to inspect the sensor connector for corrosion or water intrusion.
- Clearing the code before recording Freeze Frame data.
- Using low-quality aftermarket oxygen sensors.
Recommended Diagnostic Tools
- Professional OBD-II Scan Tool with Live Data
- Digital Multimeter (DMM)
- Automotive Oscilloscope
- Fuel Pressure Gauge
- Back Probe Test Leads
- OEM Wiring Diagram
Related Diagnostic Trouble Codes
- P0136 – O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
- P0137 – O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
- P0140 – O2 Sensor No Activity Detected (Bank 1 Sensor 2)
- P0141 – O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
- P0172 – System Too Rich (Bank 1)
Frequently Asked Questions (FAQ)
Can I continue driving with P0138?
Yes, in many cases the vehicle remains drivable. However, driving for an extended period with this fault may increase fuel consumption and potentially damage the catalytic converter.
Does P0138 always mean the oxygen sensor is bad?
No. Wiring faults, a short to voltage, poor grounding, or a rich fuel mixture can also trigger the code.
Can a bad catalytic converter cause P0138?
It is possible, but much less common than a faulty oxygen sensor, damaged wiring, or an engine running excessively rich.
Conclusion
The P0138 code indicates that the downstream oxygen sensor on Bank 1 Sensor 2 is reporting an abnormally high voltage for an extended period. The most common causes include a failed oxygen sensor, damaged wiring, or an overly rich air-fuel mixture. A proper diagnosis using live data, electrical testing, and visual inspection will identify the root cause and help prevent unnecessary parts replacement.