O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
The P0139 code means the vehicle's onboard diagnostic system has detected: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2). This is a low severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- moderate
- Estimated cost
- $80 - $300 (O2 sensor $60-$200; labor $50-$150)
Test O2 sensor voltage (should fluctuate 0.1-0.9V). Do not replace a $150 sensor without confirming it is actually bad first.
Shows real-time O2 sensor readings on your phone. See if the sensor is lazy, stuck, or actually working before spending money.
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Common Symptoms
- Check Engine Light on
- Slightly reduced fuel economy
- Possible slight increase in emissions
- No noticeable drivability issues
- Catalyst monitor may not complete
Probable Causes (Ranked by Likelihood)
- Aging downstream O2 sensor with slow response time Very High
- Exhaust leak before the downstream sensor High
- Contaminated O2 sensor from oil or coolant in exhaust Moderate
- Wiring issue causing degraded signal Moderate
- Failing catalytic converter affecting sensor readings Low
Common Fixes by Vehicle
What techs usually find when diagnosing P0139 on specific vehicles — tap your vehicle for the fix and the exact part:
2004-2014 Ford F-150/Explorer 4.6L/5.4L/4.0L Easy (sensor), Moderate (cat) DIY
Downstream O2 slow response — catalytic converter efficiency dropping. Replace the downstream O2 sensor first (much cheaper). If code returns in 100 miles, the catalytic converter needs replacement. Only use direct-fit cats on Ford V8s.
Labor: 30-45 min (sensor), 2-3 hr (cat)2003-2011 Honda Accord/Civic/CR-V Easy DIY
Downstream O2 slow response on Honda — replace with an OE-quality Denso unit OEM sensor. Verify no exhaust leak upstream, as a small leak dramatically slows downstream sensor response. Aftermarket O2 sensors commonly re-trigger this code.
Labor: 30 min2005-2014 Toyota Camry/Tundra/Tacoma Easy DIY
Slow response post-cat on Toyota — often a sign of impending cat failure. Replace O2 first. If cat is failing, confirm no underlying misfire that damaged the cat. Toyota OEM cats are expensive; CA-spec cats are required in California.
Labor: 30-45 minFrequently Asked Questions
Can I drive with code P0139?
Yes, you can drive normally. The downstream O2 sensor monitors catalytic converter efficiency. A slow response will not affect engine performance but may indicate the sensor or catalyst is aging.
Could P0139 actually be a catalytic converter problem?
Possibly. A deteriorating catalytic converter can cause the downstream O2 sensor to show unusual patterns that mimic a slow sensor. If replacing the sensor does not fix the code, have the catalytic converter tested.
How long do O2 sensors last?
O2 sensors typically last 60,000 to 100,000 miles. The downstream sensor often lasts longer because it is exposed to cleaner exhaust after the catalytic converter. If your vehicle has high mileage, replacing the sensor is a reasonable first step.
What actually makes the computer set an oxygen sensor code?
The OBD II regulation requires four separate detections for an oxygen sensor (Title 13 CCR 1968.2, section (e)(7.2)): deterioration in voltage, response rate or amplitude that would push emissions past 1.5 times the FTP standards; loss of circuit continuity; out-of-range values; and the case where the sensor has degraded enough that the fuel system stops using it and falls back to open loop. A sensor can therefore set a code while still producing a signal that looks plausible on a scan tool. This is the threshold the monitor is designed around, not a description of your specific fault — the regulation says what the system must detect, not how to find it on your vehicle.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
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Use a scan tool to monitor bank 1 sensor 2 O2 voltage during a steady cruise at 2,000 RPM. The downstream sensor should show a relatively steady voltage (0.5-0.7V) if the catalyst is working. Slow transitions indicate a lazy sensor.
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Compare the downstream sensor switching rate to the upstream sensor. The downstream should switch much less frequently than the upstream if the catalyst is functioning.
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Check for exhaust leaks between the catalytic converter and the downstream sensor location. Use a smoke machine or visual inspection.
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Inspect the O2 sensor for contamination (white residue = coolant, oily residue = burning oil).
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Check wiring and connector for corrosion, damage, or high resistance that could slow the signal.
Sources 3
This page is built from documented references. Verify against your own service info before repair work.
- California Code of Regulations, Title 13 section 1968.2 (OBD II)malfunction criteria, section (e)
- SAE J2012Diagnostic Trouble Code Definitions (defines the generic OBD-II code ranges)
- Cross-checked against the SAE J2012:2002 DTC table (2026-08-27)archived scan of the SAE standard — readable copy, not SAE’s current edition
Commonly Affected Vehicles
- 2005-2015 Toyota Camry (2.5L)
- 2007-2016 Honda CR-V (2.4L)
- 2006-2014 Nissan Altima (2.5L)
- 2008-2017 Subaru Outback (2.5L)
- 2010-2018 Ford Fusion (2.5L)
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