| DTC Data Sheet | |
| System | Powertrain |
| Standard | ISO/SAE Controlled |
| Fault type | Circuit High | Location: Bank 1, Sensor 2 |
| Official meaning | O2 Sensor Circuit High Voltage Bank 1 Sensor 2 |
DTC P0138 – O2 Sensor Circuit High Voltage Bank 1 Sensor 2 is an ISO/SAE controlled powertrain code that sets when the PCM/ECM detects the Bank 1 Sensor 2 oxygen sensor circuit voltage is higher than expected for a calibrated period of time. Bank 1 is the engine side that contains cylinder 1, and Sensor 2 is the oxygen sensor located after the catalytic converter. Because this is a circuit high fault, the diagnostic focus is electrical: the signal circuit may be held high by a wiring short to voltage, a ground/return issue that elevates the reading, connector problems, or an internal sensor fault. The vehicle may still feel normal, but emissions monitoring and readiness can be affected.
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P0138 Quick Answer
P0138 – O2 Sensor Circuit High Voltage Bank 1 Sensor 2 means the PCM/ECM is seeing the Bank 1 Sensor 2 O2 sensor circuit voltage higher than it should be. Start by inspecting the Bank 1 Sensor 2 connector and harness near the exhaust, then test for a short-to-voltage or ground/return problem that can keep the signal high.
What Does P0138 Mean?
Official meaning: O2 Sensor Circuit High Voltage Bank 1 Sensor 2. This indicates the PCM/ECM has detected an abnormally high voltage condition on the Bank 1 Sensor 2 oxygen sensor signal circuit (the downstream sensor after the catalytic converter on the bank that contains cylinder 1).
“Circuit High” describes what the control module measures electrically on the O2 sensor circuit, not a guaranteed conclusion about the exhaust mixture. In other words, P0138 tells you the voltage the module is receiving from the Bank 1 Sensor 2 circuit is too high for the monitor’s criteria; it does not, by itself, prove the sensor is bad. Accurate diagnosis requires checking the circuit, connector integrity, and sensor operation under the conditions present in freeze-frame data.
Theory of Operation
Bank 1 Sensor 2 is positioned downstream of the catalytic converter and provides feedback used primarily for catalyst monitoring and downstream O2 sensor monitoring strategies. The PCM/ECM evaluates the downstream sensor signal for plausibility and expected behavior over time. Under typical operating conditions, the downstream O2 sensor voltage should not remain at an excessively high level beyond what the monitor allows.
P0138 sets when the module’s diagnostic logic determines the Bank 1 Sensor 2 circuit voltage is above a specified threshold long enough to be considered a fault. A true electrical “high” can be created by a short-to-voltage on the signal circuit, unintended cross-feed from another powered circuit, or a compromised ground/return path that alters the measured voltage. An internal sensor fault can also bias the output high and produce the same result at the PCM/ECM input.
Symptoms
- Malfunction indicator lamp (MIL) illuminated
- Stored DTC P0138 (pending and/or confirmed), often with freeze-frame data captured
- Emissions or inspection failure due to the MIL or incomplete readiness after clearing codes
- Live data showing Bank 1 Sensor 2 voltage reading high and/or remaining high longer than expected
- Potential additional O2 sensor or catalyst-monitor related codes depending on what the PCM/ECM detects
Common Causes
- Bank 1 Sensor 2 signal circuit shorted to voltage
- Damaged wiring insulation from heat exposure or contact with exhaust components causing unwanted voltage on the signal line
- Connector damage (moisture intrusion, corrosion, terminal spread, or poor pin fit) at Bank 1 Sensor 2
- High resistance or open in the sensor signal return/ground circuit causing an elevated measured voltage
- Bank 1 Sensor 2 oxygen sensor internal fault causing a biased high output
- Incorrect wiring or incorrect sensor application resulting in improper circuit behavior and high voltage at the PCM/ECM input
- PCM/ECM input circuit fault (diagnose only after verifying wiring and sensor integrity)
Diagnosis Steps
Tools needed: scan tool with live data (and readiness status), DVOM (digital volt/ohm meter), back-probing pins or approved test leads, and the correct wiring diagram/service information for the exact vehicle configuration. A test light or fused jumper can be useful for loading circuits during ground/return checks.
- Confirm the code. Use a scan tool to verify P0138 is present (pending and/or confirmed). Record freeze-frame data (RPM, load, coolant temperature, battery voltage, fuel system status) and note any additional DTCs that may affect O2 sensor operation or monitoring.
- Identify the correct sensor. Confirm the physical location of Bank 1 Sensor 2 (downstream/post-catalyst on the bank that contains cylinder 1). Ensure you are testing the correct connector and harness.
- Perform a focused visual inspection. Inspect the Bank 1 Sensor 2 harness routing for melted insulation, abrasion, crushed sections, or contact with hot exhaust components. Check for signs of pulling tension at the pigtail, improper routing, or missing clips that could allow chafing.
- Inspect the connector and terminals. Disconnect the Bank 1 Sensor 2 connector and examine for moisture, corrosion, damaged seals, bent pins, terminal spread, or evidence of overheating. Correct any obvious connector issues before deeper electrical testing.
- Check live data behavior. With the engine warmed to normal operating temperature (when safe and appropriate), monitor the Bank 1 Sensor 2 voltage PID. Compare the reading to what the PCM/ECM is flagging as “high” in freeze-frame conditions. Note whether the voltage appears fixed high or changes at all with normal engine operation.
- Key OFF, isolate the sensor. Turn the key OFF and disconnect Bank 1 Sensor 2. This separates the sensor from the harness and helps determine whether the high voltage is being generated by the sensor or by the vehicle wiring/PCM input.
- Key ON, test for short-to-voltage on the signal circuit. With the sensor still unplugged and ignition ON, use the wiring diagram to identify the O2 signal pin at the harness side. Measure voltage from the signal pin to chassis ground. A steady voltage present with the sensor disconnected strongly indicates the circuit is being driven high by a short-to-voltage, cross-feed, or a PCM/ECM input bias issue.
- Test signal return/ground integrity. Using service information to identify the signal return/ground circuit, perform a voltage-drop test under load (as specified by the service procedure). Excessive voltage drop indicates resistance in the return path (connector, splice, ground point, or harness) that can distort the signal and contribute to a “circuit high” reading.
- Check for cross-feed between circuits. With the connector unplugged, use the wiring diagram to identify adjacent circuits and any powered feeds. Verify there is no unintended continuity between the signal terminal and any power feed terminals. Manipulate (wiggle) the harness during testing to reveal intermittent contact or insulation breakdown.
- Verify repair and monitor completion. After correcting the identified fault, clear codes, then run the vehicle under conditions that allow OBD-II monitors to run. Recheck for pending/confirmed P0138 and confirm readiness status completes as applicable.
Professional tip: If the scan tool (or a voltage measurement at the harness) still shows the Bank 1 Sensor 2 signal high with the sensor unplugged, the sensor itself cannot be the source of that voltage. In that case, prioritize diagnosing a short-to-voltage, cross-feed, connector issue, or PCM/ECM input problem before replacing the sensor.
Possible Fixes
- Repair or replace damaged wiring/loom and reroute the harness away from heat sources and moving parts
- Repair connector issues (clean/dry moisture intrusion, address corrosion, restore terminal tension, replace damaged seals or pigtail as required)
- Correct any short-to-voltage or cross-feed condition affecting the Bank 1 Sensor 2 signal circuit
- Repair high-resistance or open in the signal return/ground circuit verified by voltage-drop testing
- Replace Bank 1 Sensor 2 oxygen sensor only after circuit testing supports an internal sensor fault
- Diagnose PCM/ECM input circuit concerns only after verifying sensor and wiring integrity end-to-end
Can I Still Drive With P0138?
In many cases the vehicle can still be driven with P0138 present because Bank 1 Sensor 2 is a downstream sensor used for monitoring functions. However, the MIL will remain on, emissions-related readiness can be affected, and the vehicle may fail inspection. If the engine runs poorly, you detect strong fuel odor, or other fault codes indicate additional problems, reduce driving and diagnose promptly because unresolved issues can increase emissions and complicate further diagnostics.
How Serious Is This Code?
P0138 is typically serious from an emissions and diagnostic standpoint because it indicates the PCM/ECM cannot rely on the Bank 1 Sensor 2 circuit signal as expected. The immediate drivability impact may be minimal, but the underlying cause could be electrical damage (such as harness contact with exhaust heat) that can worsen over time. Because P0138 is a circuit-high code, confirming wiring integrity and connector condition is important to prevent repeat faults and ensure the powertrain monitoring system operates correctly.
Common Misdiagnoses
The most frequent misdiagnosis is replacing Bank 1 Sensor 2 without proving the circuit is not being held high by a wiring short-to-voltage, connector moisture/corrosion, or a compromised return/ground. Another mistake is ignoring freeze-frame conditions and failing to test with the sensor unplugged, which is a key step to distinguish a sensor-generated voltage from a harness/PCM-driven high reading. Misidentifying sensor location (testing the wrong bank or wrong sensor number) can also lead to wasted parts and unchanged results.
Most Likely Fix
The most likely corrective action is to restore proper electrical integrity of the Bank 1 Sensor 2 circuit by repairing wiring or connector faults that hold the signal voltage high (such as heat-damaged insulation, a short-to-voltage, or terminal/connector problems). Replace the oxygen sensor only after tests show the circuit is not being forced high externally and the sensor output remains biased high under verified conditions.
Repair Costs
Hybrid and EV high-voltage system repairs require certified technicians and specialist equipment. Costs vary widely depending on whether the fault is wiring, a sensor, a module, or a high-voltage assembly.
| Repair Type | Estimated Cost |
|---|---|
| Specialist diagnosis (HV-certified technician) | $150 – $300 |
| HV wiring / connector / sensor repair | $150 – $800+ |
| HV module / inverter / battery management repair | $500 – $3000+ |
| HV battery pack replacement | $2000 – $8000+ |
Brand-Specific Guides for P0138
Manufacturer-specific diagnostic procedures with factory data and pin-level details for vehicles where this code commonly sets:
Key Takeaways
- P0138 – O2 Sensor Circuit High Voltage Bank 1 Sensor 2 is an ISO/SAE controlled powertrain DTC indicating a circuit-high voltage condition on the downstream O2 sensor circuit.
- “Circuit High” points to an electrical measurement that can be caused by wiring shorts, connector issues, ground/return problems, or an internal sensor fault.
- Unplugging Bank 1 Sensor 2 and checking for voltage on the harness-side signal circuit helps separate sensor faults from wiring/PCM-driven high voltage.
- Correct diagnosis relies on wiring diagrams, DVOM tests (including voltage-drop), and confirmation using freeze-frame data and live data.
- Clearing codes alone is not verification; confirm P0138 does not return and that monitors can complete as applicable.
FAQ
What is the official meaning of P0138?
The official meaning of P0138 is O2 Sensor Circuit High Voltage Bank 1 Sensor 2.
Which oxygen sensor does P0138 refer to?
P0138 refers to Bank 1 Sensor 2, the downstream oxygen sensor located after the catalytic converter on the engine bank that contains cylinder 1.
Does P0138 always mean the O2 sensor is bad?
No. P0138 indicates the circuit voltage is high at the PCM/ECM input for Bank 1 Sensor 2. A short-to-voltage, connector contamination, or a return/ground problem can hold the signal high and mimic a failed sensor, so circuit testing should come before parts replacement.
What is the fastest initial test for a circuit-high condition on Bank 1 Sensor 2?
A practical first discriminator is to disconnect Bank 1 Sensor 2 and then check whether the signal remains high (using scan tool data and/or a DVOM at the harness). If the reading remains high with the sensor unplugged, the high voltage is being introduced by the harness, cross-feed, or PCM/ECM input circuit rather than the sensor itself.
How do I confirm the repair for P0138 is complete?
After repairs, clear DTCs and recheck that P0138 does not return as pending or confirmed. Then verify live data looks plausible for Bank 1 Sensor 2 and confirm readiness status can complete as applicable, since clearing codes resets monitor status.