P0138: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
P0138 means the rear oxygen sensor on bank 1 is reporting a voltage that is too high for too long, usually because the sensor is biased rich or the wiring is shorted. The most common fix is replacing the Bank 1 Sensor 2 O2 sensor or repairing its wiring, typically $150 to $350 at an independent shop.
- Severity
- Moderate
- Can I drive?
- Drive with caution
- DIY?
- Possible with some experience
- Repair cost range
- $100 – $400
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What does P0138 mean?
This code points to the downstream oxygen sensor on bank 1, which is the sensor after the catalytic converter on the engine side that contains cylinder #1. That sensor helps the engine computer monitor catalytic converter performance and exhaust oxygen content, not normally fuel trim control. When the sensor voltage stays high, the computer thinks the exhaust is staying rich or the sensor circuit is being held at a high signal. The problem can come from a bad sensor, damaged wiring, an exhaust leak issue near the sensor, or an engine condition that is actually running rich. On many vehicles, the code turns on the check engine light but the car may still drive normally at first.
Common symptoms
- Check engine light on
- Possible rotten egg smell from exhaust
- Poor fuel economy
- Rough running if the engine is actually running rich
- Failed emissions test
- Sometimes no noticeable drivability problem
Causes, fixes and costs
P0138: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
General guide for most cars. Your car may differ.
This code is usually not an immediate breakdown risk, but it can hide a real rich-running problem or cause the vehicle to fail an emissions test. Stop driving if the engine runs badly, the exhaust smells strongly of fuel, or the check engine light starts flashing.
Most likely causes
- 1
Faulty Bank 1 Sensor 2 oxygen sensorHigh
A worn or contaminated downstream O2 sensor can report a constant high voltage even when the exhaust mixture is normal. This is most likely if the code is the only symptom and the car otherwise drives normally.
Quick check: Look at live data with a scan tool; if the sensor voltage stays stuck high and does not switch normally, suspect the sensor.
- 2
Shorted or damaged sensor wiringMedium
Heat damage near the exhaust or a rubbed-through wire can hold the signal high and set P0138. This is more likely if the code comes and goes or appears after work was done near the exhaust.
Quick check: Inspect the sensor harness, connector, and wiring near the exhaust for melted insulation, corrosion, or loose pins.
- 3
Engine running richMedium
If the engine is actually getting too much fuel, the downstream sensor may see rich exhaust for an extended period. Look for poor mileage, fuel smell, black smoke, or a rough idle along with the code.
Quick check: Check for rich-running clues such as fuel odor at the tailpipe, black soot, or other fuel-system codes.
- 4
Exhaust contamination or coolant/oil entering exhaustLow
Oil burning or coolant intrusion can contaminate the sensor and skew its reading high. This is more likely if there is blue smoke, white smoke, or other engine problems along with the code.
Quick check: Inspect for smoke, coolant loss, or oil consumption that could foul the sensor.
Can you fix it yourself?
Possible with some experience
Difficulty: Moderate. A handy owner can inspect wiring and, on some cars, replace the rear O2 sensor with basic hand tools and a scan tool. You need the correct sensor for Bank 1 Sensor 2 and may need an O2 sensor socket, penetrating oil, and access from underneath the vehicle.
Show the stepsHide the steps
- Verify the code and check live sensor data.
- Inspect the connector and wiring for heat damage or corrosion.
- Repair any damaged wiring or loose connection.
- If wiring is good, replace Bank 1 Sensor 2 with the correct part.
- Clear the code and road test the vehicle.
- Recheck for returning codes and confirm the sensor now switches normally.
Estimated repair cost
$150 – $350
Bank 1 Sensor 2 oxygen sensor replacement: parts ~$50-$150, labor ~$100-$200
- O2 sensor wiring repair$100 – $250
- Exhaust leak repair near sensor$120 – $400
- Fuel system diagnosis for rich condition$120 – $220
Prices vary by vehicle, engine layout, rust, and local labor rates.
Frequently asked questions
Can I drive with P0138?
Usually yes for a short time, but you should get it checked soon. If the engine runs rough, smells strongly of fuel, or the check engine light flashes, stop driving.
Will P0138 fail an emissions test?
It often will, because the check engine light is on and the monitor may not run correctly. Even if the car drives fine, a stored P0138 is a common inspection failure.
Is the oxygen sensor always the problem?
No. The sensor is the most common cause, but wiring problems or a real rich-running engine can set the same code. A quick live-data check helps separate those causes.
Can I clear P0138 and ignore it?
You can clear it, but it usually comes back if the fault is still there. Ignoring it can waste fuel and may hide a bigger fuel or exhaust problem.
Is Bank 1 Sensor 2 the front or rear O2 sensor?
It is usually the rear, downstream sensor on the side of the engine with cylinder #1. It sits after the catalytic converter, not before it.
Related codes
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This guide covers most cars and is general advice, not an inspection. Prices are typical US independent-shop ranges and vary by region, car and shop. If your brakes, steering or a warning light feel unsafe, stop driving and contact a mechanic.