Faulty downstream oxygen sensor (both sources list this first). Signal wire shorted to battery voltage, or other wiring and connector faults in the sensor circuit. Engine actually running rich: OBD-Codes.com mentions high fuel pressure (not as likely); RepairPal lists low fuel pressure instead, along with a defective coolant temperature sensor. Exhaust leak or intake/vacuum leak (RepairPal). Defective catalytic converter (RepairPal). Oxygen sensor heater circuit fault (RepairPal). Less often, PCM software that needs updating or a faulty PCM (RepairPal).

Replace the Bank 1 Sensor 2 oxygen sensor if testing shows it's faulty. Repair a short to voltage or other damage in the sensor signal wiring. Fix the cause of a rich mixture (fuel pressure, coolant temperature sensor, leaks) if the engine really is running rich. Repair exhaust leaks ahead of the sensor. Apply any PCM software update the manufacturer has issued for this code.

P0138🌫 Emissions / oxygen sensorsDrive with caution, fix soon

P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2) (Meaning, Causes, Fixes)

Q: What does P0138 mean?

P0138 means the downstream oxygen sensor (after the catalytic converter) on Bank 1 is reporting a voltage that stays too high for too long. A faulty sensor is the most common cause; a short to battery voltage in the signal wire and fuel-mixture problems are the next things to check. Sensor 2 is the downstream oxygen sensor, mounted after the catalytic converter on Bank 1 (the side of the engine with cylinder #1; a 4-cylinder has only Bank 1). Its job is to watch how well the converter is storing oxygen, so its signal normally moves much less than the front sensor's. A high voltage reading from this kind of sensor means the exhaust looks rich (low in oxygen). The sources describe the trigger differently: OBD-Codes.com says the code sets when the sensor voltage stays above 999 mV for more than about 2 minutes (up to 4 minutes on some models), while RepairPal says it sets when the voltage stays above 450 mV for more than about twenty seconds, or when an air-fuel ratio sensor stays rich-biased too long. Both say the exact threshold and time vary by make and model. P0137 is the matching low-voltage code for the same sensor, and P0132 is the high-voltage code for the upstream sensor.

Q: Can I keep driving with P0138?

Drive with caution, fix soon. Usually drivable. RepairPal rates it Moderate: not an immediate safety threat, but get it diagnosed within a reasonable time. OBD-Codes.com says there may be no symptoms beyond the check engine light, but an open code can fail an OBD2 emissions inspection, so don't leave it until inspection day.

Reviewed 2026-10-22 · Based on 2 cited source(s)

What P0138 means

Sensor 2 is the downstream oxygen sensor, mounted after the catalytic converter on Bank 1 (the side of the engine with cylinder #1; a 4-cylinder has only Bank 1). Its job is to watch how well the converter is storing oxygen, so its signal normally moves much less than the front sensor's. A high voltage reading from this kind of sensor means the exhaust looks rich (low in oxygen). The sources describe the trigger differently: OBD-Codes.com says the code sets when the sensor voltage stays above 999 mV for more than about 2 minutes (up to 4 minutes on some models), while RepairPal says it sets when the voltage stays above 450 mV for more than about twenty seconds, or when an air-fuel ratio sensor stays rich-biased too long. Both say the exact threshold and time vary by make and model. P0137 is the matching low-voltage code for the same sensor, and P0132 is the high-voltage code for the upstream sensor.

Severity: can I keep driving?

Drive with caution, fix soon

Usually drivable. RepairPal rates it Moderate: not an immediate safety threat, but get it diagnosed within a reasonable time. OBD-Codes.com says there may be no symptoms beyond the check engine light, but an open code can fail an OBD2 emissions inspection, so don't leave it until inspection day.

Common symptoms

Check engine light on (OBD-Codes.com says this may be the only sign)
Rough idle or rough running (RepairPal)
Lower fuel economy (RepairPal)
Bad-smelling exhaust (RepairPal)
May fail an OBD2 emissions inspection

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Faulty downstream oxygen sensor (both sources list this first)
Signal wire shorted to battery voltage, or other wiring and connector faults in the sensor circuit
Engine actually running rich: OBD-Codes.com mentions high fuel pressure (not as likely); RepairPal lists low fuel pressure instead, along with a defective coolant temperature sensor
Exhaust leak or intake/vacuum leak (RepairPal)
Defective catalytic converter (RepairPal)
Oxygen sensor heater circuit fault (RepairPal)
Less often, PCM software that needs updating or a faulty PCM (RepairPal)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes and freeze-frame data. If P0138 shows up with rich-mixture codes such as P0172 or P0175, or with misfire codes, fix those first, since a truly rich engine can push the rear sensor high.
If P0132 (front sensor high voltage) is also set, suspect a shared wiring or power problem before buying two sensors.
Find the Bank 1 Sensor 2 sensor (after the catalytic converter) and inspect its wiring and connector for melted or chafed insulation, wires rubbing on the exhaust, corrosion or a loose plug.
Check for exhaust leaks ahead of the sensor and for intake or vacuum leaks.
With a scan tool that shows live data, watch the rear sensor voltage on a warm engine. A reading pinned high while the front sensor switches normally points at the rear sensor or its circuit.
Clear the code and drive under the conditions in the freeze-frame data to see whether it returns.

Typical fixes

Replace the Bank 1 Sensor 2 oxygen sensor if testing shows it's faulty
Repair a short to voltage or other damage in the sensor signal wiring
Fix the cause of a rich mixture (fuel pressure, coolant temperature sensor, leaks) if the engine really is running rich
Repair exhaust leaks ahead of the sensor
Apply any PCM software update the manufacturer has issued for this code

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

The code comes back after a new sensor (OBD-Codes.com's forums have many such cases, often pointing to wiring, fuel pressure or the PCM)
You need a fuel-pressure test or circuit diagrams to trace the signal wire
The sensor is seized in the exhaust
The car runs noticeably rich, rough or smells strongly of fuel

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is P0138 serious?

Usually not an emergency. RepairPal rates it Moderate, and OBD-Codes.com says there may be no symptoms other than the check engine light. It's still worth fixing soon because the car may be running rich and can fail an emissions test.

Will a new oxygen sensor fix P0138?

Often, since both sources list a faulty sensor first. But a short to voltage in the wiring, or an engine that really is running rich, will set the same code, so check the wiring and fuel mixture before buying parts.

Why do sources give different voltages for P0138?

OBD-Codes.com describes the code setting above 999 mV for about 2 to 4 minutes; RepairPal describes above 450 mV for about twenty seconds. Both say the exact limits depend on the make and model, so your service manual has the real numbers for your car.

High or low fuel pressure?

The sources disagree. OBD-Codes.com lists high fuel pressure (as a less likely cause), while RepairPal lists low fuel pressure. A fuel-pressure test against your vehicle's specification settles it.

Sources

Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.