P0139
Fix within 1-2 weeksYour downstream oxygen sensor (Bank 1, Sensor 2) is responding too slowly to changes in the exhaust gas.
Code P0139 means your downstream oxygen sensor (Bank 1, Sensor 2) is responding too slowly. Common causes: Faulty O2 sensor (80%), exhaust leak (10%), wiring issues (8%). Drivability: Yes, safe to drive. Repair cost: $50-$200 DIY, $150-$500 professional.
Can I Drive With P0139?
Yes, safe to drive
No restrictions
Stop driving immediately if: Flashing Check Engine Light (indicating a misfire), loud exhaust noises, or a strong rotten egg smell from the exhaust.
Symptoms of P0139
- 1Check Engine Light is on
- 2Decreased fuel economy
- 3Vehicle fails emissions test
Common Causes of P0139
Parts most likely causing this code, ranked by probability
The oxygen sensor itself has degraded over time and can no longer respond quickly enough to changes in exhaust gas composition.
80% of P0139 cases are fixed by replacing this part
An exhaust leak near the oxygen sensor can introduce outside air, causing the sensor to read incorrectly or respond slowly.
10% of P0139 cases are fixed by repairing an exhaust leak
Damaged, corroded, or loose wiring and connectors to the oxygen sensor can disrupt its signal and cause a slow response.
8% of P0139 cases are fixed by repairing wiring
Exposure to antifreeze, oil, or certain fuel additives can foul the sensor, making it sluggish or unresponsive.
2% of P0139 cases are fixed by addressing contamination
Check These First (Before Buying Parts)
- Visually inspect the Bank 1 Sensor 2 oxygen sensor and its wiring for any obvious damage, fraying, or corrosion.
- Listen for any exhaust leaks, especially under the vehicle near the sensor. A hissing sound indicates a leak.
How to Diagnose P0139
- 1Scan for any other related codes to rule out other issues.
- 2Visually inspect the Bank 1 Sensor 2 oxygen sensor and its wiring harness for any signs of damage, corrosion, or loose connections.
- 3Check for exhaust leaks, especially upstream of the sensor, by listening for hissing sounds or using a smoke machine.
- 4Use a scan tool to monitor the live data of the Bank 1 Sensor 2 oxygen sensor, observing its voltage fluctuations and response time.
- 5If the sensor's response is consistently slow, replace the Bank 1 Sensor 2 oxygen sensor.
P0139 on Popular Vehicles
Toyota Camry
2007-2024P0139 on Toyota Camry models is commonly attributed to the downstream oxygen sensor degrading over time, often after 100,000 miles. Replacement of the sensor is a direct fix.
85% fixed by replacing the O2 sensor
Honda Accord
2003-2024Honda Accord vehicles frequently experience P0139 due to the natural lifespan of the downstream oxygen sensor. Visual inspection for exhaust leaks is always a good first step, but sensor replacement is the most common solution.
80% fixed by replacing the O2 sensor
Ford F-150
2004-2024For Ford F-150 trucks, P0139 is typically a result of a failing downstream oxygen sensor. While exhaust leaks are possible, especially around the manifold, the sensor itself is the primary suspect for this specific code.
75% fixed by replacing the O2 sensor
Frequently Asked Questions
What does P0139 mean?
P0139 indicates your vehicle's downstream oxygen sensor (Bank 1, Sensor 2) is responding too slowly to changes in exhaust gas composition.
Can I drive with P0139?
Yes, you can drive safely. However, fix this within 1-2 weeks to prevent decreased fuel economy and potential emissions test failure.
Will P0139 clear itself?
No, P0139 will not clear itself. The underlying issue, usually a faulty O2 sensor, requires repair to resolve the code.
How do I fix P0139 myself?
Start by visually inspecting the O2 sensor and its wiring for damage or corrosion. Check for exhaust leaks. If no issues are found, replacing the Bank 1 Sensor 2 oxygen sensor is the next step.
How much does it cost to fix P0139?
DIY repair costs $50-$200 for the O2 sensor. Professional repair costs $150-$500, including parts and labor.
P0139 keeps coming back - why?
The most common reason for recurrence is not replacing the correct sensor, or an underlying exhaust leak or wiring issue was missed during the initial repair.
Will P0139 fail emissions/smog test?
Yes, P0139 will cause your vehicle to fail an emissions or smog test because it indicates a problem with the emissions monitoring system.
What's the difference between P0139 and P0420?
P0139 specifically indicates a slow response from the downstream O2 sensor itself, while P0420 indicates the catalytic converter is not working efficiently enough. A faulty O2 sensor can sometimes trigger P0420, but P0139 points directly to the sensor.
P0139 on Toyota Camry?
On Toyota Camry models, P0139 is frequently caused by the downstream oxygen sensor simply failing due to age and mileage, typically after 100,000 miles.
Questions to Ask Your Mechanic
Did you check the live data for the Bank 1 Sensor 2 oxygen sensor's voltage response?
Good answer: Yes, the sensor's voltage was fluctuating too slowly, indicating it's sluggish and needs replacement.
Have you thoroughly checked for any exhaust leaks near the downstream O2 sensor?
Good answer: Yes, I performed a visual inspection and a smoke test, and there are no detectable exhaust leaks.
Repair Cost Estimate
DIY Repair
Parts only
$50-$200
Shop Repair
Parts + labor
$150-$500
Save $100-$300 by doing it yourself
Difficulty
intermediate
Time
1-2 hours
Intermediate - requires specific tools like an O2 sensor socket and potentially a scan tool for proper diagnosis, but the physical replacement is manageable for many DIYers.
Cost of Waiting
Ignoring P0139 leads to decreased fuel efficiency and your vehicle failing emissions tests. While it won't cause immediate engine damage, prolonged poor fuel trim can indirectly affect other emissions components over time. A $50-$150 O2 sensor replacement now prevents ongoing fuel waste.
While You're In There...
If you need to remove exhaust components to access the sensor, replacing old gaskets prevents future leaks and ensures a proper seal.
Applying anti-seize to the new O2 sensor threads prevents it from seizing in the exhaust pipe, making future removal easier.
Tools You'll Need
- OBD-II Scan Tool
- O2 Sensor Socket
- Ratchet
- Jack and Jack Stands (or vehicle lift)
- Penetrating Oil
- Wire Brush (optional, for cleaning threads)
P0139 is a common code, often indicating a simple sensor replacement. Most repairs are straightforward, and your vehicle's fuel economy and emissions performance will improve afterward.
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Your Next Steps
Step 1: Visually inspect the O2 sensor and its wiring for any damage or loose connections. Check for exhaust leaks.
Step 2: If no obvious issues are found, use an OBD-II scan tool to monitor the live data of the Bank 1 Sensor 2 O2 sensor. Look for slow or erratic voltage readings.
Step 3: If the sensor is confirmed to be sluggish, replace the Bank 1 Sensor 2 oxygen sensor. You'll likely need an O2 sensor socket for this.