P0138 Code: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
P0138 indicates high voltage in the Bank 1 Sensor 2 O2 circuit. Don't just replace the sensor; diagnose wiring shorts, connector issues, or rich fuel conditions first. Learn proper location, symptoms, and step-by-step fixes to avoid costly mistakes.
Quick Answer
P0138 means the ECM detected excessively high voltage in the Bank 1 Sensor 2 oxygen-sensor circuit. Sensor 2 is the downstream sensor located after the catalytic converter on the side containing cylinder 1. The code does not prove the sensor is bad. A failed sensor, melted or shorted signal wire, contaminated connector, poor signal ground, or genuinely rich exhaust can produce the same result. Locate the correct sensor, inspect its wiring, compare downstream voltage with upstream data and fuel trims, and confirm the fault before replacing parts.
Safety note: Exhaust components and oxygen sensors become hot enough to cause serious burns. Let the exhaust cool before inspecting it. Use properly rated lifting equipment on approved lift points if underbody access is required. Do not test wiring by piercing insulation or applying external voltage unless the vehicle’s service procedure specifically instructs you to do so.
What Does the P0138 Code Mean?
P0138 is a generic OBD-II code defined as O2 Sensor Circuit High Voltage, Bank 1 Sensor 2. The ECM stores it when the signal from the downstream oxygen sensor remains above the calibrated limit for a specified period.
The exact voltage threshold, enabling conditions, and time requirement vary by vehicle. A scanner description that says “circuit high” does not automatically mean the exhaust is rich, and it does not confirm that the sensing element has failed.
- Bank 1 is the engine bank containing cylinder 1. An inline engine has only one bank.
- Sensor 2 is the sensor positioned downstream of the first catalytic converter for that bank.
- Circuit high voltage means the ECM saw an electrical signal above the expected range or saw a high signal for too long.
Sensor 1 primarily helps the ECM manage the air-fuel mixture. Sensor 2 mainly monitors the exhaust after it passes through the converter. Some control strategies may use downstream data for limited fuel-trim correction, but its primary job is catalyst monitoring.
Where Is Bank 1 Sensor 2 Located?
Bank 1 Sensor 2 is threaded into the exhaust system after the catalytic converter on the bank containing cylinder 1. Start at the engine and follow the Bank 1 exhaust path. Sensor 1 appears before the converter; Sensor 2 appears after it.
Its exact position depends on the exhaust layout:
- On many inline four-cylinder engines, it is located in the exhaust pipe immediately behind the converter.
- On V-type engines, identify which side contains cylinder 1 before choosing a sensor.
- Some vehicles have more than one converter or additional monitoring sensors, so “the rear sensor” is not always a precise identification.
- Transverse engines can place Bank 1 at the front or rear of the engine compartment depending on the engine design.
Do not identify Bank 1 by assuming it is always the driver or passenger side. Confirm the cylinder layout through the underhood emissions label or vehicle-specific service information. Before ordering, also verify the listing’s installation position, connector, wire length, sensor technology, and engine application.
How Serious Is P0138, and Can You Drive With It?
P0138 is usually a moderate-priority fault. If the check-engine light is steady and the engine starts and runs normally, limited driving while arranging diagnosis is generally possible. A downstream sensor circuit fault by itself may cause no noticeable drivability symptoms.
The risk is higher when P0138 reflects a genuinely rich-running engine rather than an isolated sensor or wiring fault. Excess fuel can increase emissions, foul spark plugs, dilute engine oil, and overheat the catalytic converter.
Stop driving and diagnose the vehicle promptly if:
- The check-engine light flashes.
- The engine misfires severely, stalls, or loses substantial power.
- Black smoke comes from the exhaust.
- You smell raw fuel or find a fuel leak.
- The catalytic converter becomes unusually hot or glows.
An active P0138 can also prevent the emissions monitors from completing and may cause the vehicle to fail an inspection. Clearing the code just before an inspection does not complete the required readiness monitors.
What Symptoms Can P0138 Cause?
The most common symptom is a steady check-engine light. Because Bank 1 Sensor 2 is primarily a catalyst-monitoring sensor, an isolated sensor or circuit fault may not noticeably change how the engine runs.
If the high reading is caused by a true rich condition, the underlying fuel problem may produce additional symptoms:
- Poor fuel economy
- Rough idle or hesitation
- Black exhaust smoke
- Fuel odor from the exhaust
- Fouled spark plugs or misfire codes
- Negative fuel-trim readings as the ECM tries to remove fuel
These symptoms should not automatically be blamed on the downstream sensor. They are evidence that the engine may actually be receiving too much fuel.
What Causes P0138?
The most useful way to organize P0138 causes is by separating an electrical high signal from a real rich exhaust condition.
| Possible Cause | Clues That Support It |
|---|---|
| Failed or contaminated Bank 1 Sensor 2 | B1S2 remains high while upstream data, fuel trims, and the opposite bank appear normal. |
| Signal wire shorted to voltage | Harness is melted against the exhaust, pinched, incorrectly routed, or damaged near a connector. |
| Poor signal ground or circuit resistance | Corrosion, water intrusion, loose terminals, or excessive resistance is present in the circuit. |
| Leaking fuel injector | Rich codes, misfires, fuel odor, abnormal trims, or one fuel-fouled cylinder accompanies P0138. |
| Excessive fuel pressure | Measured pressure exceeds the vehicle specification and more than one cylinder appears rich. |
| Upstream air-fuel sensor or control fault | Manufacturer-specific data or an applicable bulletin directs diagnosis upstream despite P0138. |
| ECM input fault | Sensor and circuit tests pass, but the scan value remains incorrect. This is uncommon and requires factory-level testing. |
What Usually Does Not Cause P0138 by Itself?
An exhaust leak usually introduces outside oxygen and tends to create a leaner or lower-voltage reading, so it is not a primary explanation for a circuit-high code. A heater failure normally produces a heater-specific code unless damaged wires have shorted into the signal circuit.
A failed catalytic converter more commonly produces an efficiency code such as P0420. Do not replace the converter because of P0138 unless separate catalyst testing confirms it has failed.
How Do You Diagnose P0138 Without Replacing the Wrong Sensor?
- Save the scan data. Record P0138, all current and pending codes, freeze-frame data, coolant temperature, engine load, upstream and downstream sensor data, and short- and long-term fuel trims.
- Confirm the correct location. Identify the bank containing cylinder 1 and locate Sensor 2 after its converter. Do not confuse it with Bank 1 Sensor 1 or Bank 2 Sensor 2.
- Inspect the harness before testing the sensor. Look for insulation melted by the exhaust, stretched wiring, missing retaining clips, contact with sharp heat shields, water intrusion, corrosion, damaged pins, and evidence of previous splicing.
- Check relevant companion codes. Heater codes suggest a shared connector or harness problem. Rich, misfire, fuel-pressure, or injector codes increase the likelihood of a genuine fueling fault.
- Compare B1S2 with upstream and fuel-trim data. A high B1S2 signal with normal upstream behavior and normal fuel trims points more strongly toward the downstream sensor or its circuit. High readings accompanied by rich upstream data and strongly negative trims require fuel-system diagnosis.
- Test the signal and ground circuits. Follow the wiring diagram and pinout for the exact vehicle. Check for a short to voltage, open or high-resistance ground, terminal damage, and unintended contact between heater and signal wires.
- Test sensor response using the factory procedure. Do not condemn the sensor from one generic voltage number. Different systems use different bias voltages, test conditions, and failure thresholds.
- Check the rich-running causes if the exhaust is genuinely rich. Test fuel pressure and evaluate injector leakage, purge flow, upstream sensor data, and other relevant engine-management inputs.
- Consider the ECM last. An ECM fault is possible but uncommon. Confirm the sensor, connector, wiring, grounds, power supplies, and applicable software information before considering module repair.
| Scan or Test Pattern | Most Useful Next Check |
|---|---|
| Only B1S2 is abnormally high | Inspect and electrically test B1S2, its connector, signal wire, and ground. |
| Both upstream and downstream data indicate rich operation | Check fuel trims, pressure, injectors, purge flow, and upstream control inputs. |
| P0138 appears with a heater-circuit code | Inspect shared wiring, connector terminals, routing, and possible contact between circuits. |
| P0138 appears with P0420 | Resolve the signal fault, then perform the prescribed catalyst-efficiency tests before replacing the converter. |
How Do You Fix P0138, and What Can It Cost?
The correct P0138 repair depends on the diagnostic result. Clearing the code or installing a downstream sensor without testing the circuit may provide only a temporary result.
| Confirmed Problem | Typical Repair | Approximate Parts Cost |
|---|---|---|
| Failed B1S2 oxygen sensor | Replace the correct downstream direct-fit sensor | About $25–$150 |
| Damaged connector or shorted wire | Repair the harness, terminals, or connector and restore safe routing | About $10–$100 for repair materials or a pigtail |
| Leaking fuel injector | Replace the confirmed faulty fuel injector or matched set when required | Varies widely by engine and quantity |
| Incorrect fuel pressure | Repair the supply fault or replace a failed fuel pressure regulator if separately serviceable | Depends on whether the regulator is separate or integrated |
These are parts-only estimates. Labor varies significantly because some downstream sensors are accessible beside the engine, while others require underbody access and may be seized by heat and corrosion.
Choose the Correct A-Premium Oxygen Sensor
If testing confirms a failed Bank 1 Sensor 2, A-Premium provides competitively priced direct-fit oxygen sensors designed with application-specific connectors and wiring. A direct-fit design avoids cutting and splicing a universal sensor into a heat-exposed circuit.
Before ordering, enter the complete year, make, model, engine, and trim. Confirm that the listing specifically includes the downstream or B1S2 position, then compare the connector shape, terminal count, wire length, heated-sensor configuration, and product notes with the removed part. A vehicle may use visually similar sensors in different positions that are not interchangeable.
A-Premium customer service can help verify fitment before purchase. Eligible products are also supported by the warranty shown on the listing and a 90-day free-return policy, reducing the risk of being left with the wrong bank, connector, or sensor position.
How Do You Confirm the P0138 Repair Worked?
After the repair, secure the wiring away from the exhaust and moving parts. Reconnect every retaining clip and confirm that the harness is not stretched or touching a heat shield.
Clear the code, bring the engine to normal operating temperature, and repeat the operating conditions recorded in the original freeze-frame data when it is safe to do so. Monitor B1S2 voltage, upstream data, and fuel trims. Confirm that P0138 does not return as a current or pending code.
If the voltage remains high after replacing the sensor, stop replacing parts. Recheck the connector, signal circuit, ground, rich-running conditions, and any applicable manufacturer bulletins. An electrical short or upstream fueling fault will remain after a new downstream sensor is installed.
FAQs
Is Bank 1 Sensor 2 upstream or downstream?
Bank 1 Sensor 2 is downstream of the first catalytic converter on the engine bank containing cylinder 1.
Does P0138 always mean a bad oxygen sensor?
No. A shorted signal wire, poor signal ground, corroded connector, wiring damage, or genuinely rich exhaust can also cause high B1S2 voltage.
Can a bad catalytic converter cause P0138?
A converter problem is not the first diagnosis for a circuit-high code. Converter efficiency faults more commonly set P0420. Test the sensor circuit and rich-running causes before evaluating the converter.
Can an exhaust leak cause P0138?
An ordinary exhaust leak usually introduces oxygen and tends to produce a leaner or lower signal. Inspect for wiring damage and a real rich condition before treating the leak as the cause of P0138.
Is P0138 the same as P0137?
No. Both refer to Bank 1 Sensor 2, but P0138 indicates circuit voltage that is too high, while P0137 indicates circuit voltage that is too low.
Will P0138 clear itself?
The warning light may turn off after the fault stops occurring, but the code can remain stored. Clearing it does not repair the sensor, wiring, or rich-running problem.
