P0139: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
Key takeaways
- P0139 means the downstream oxygen sensor on bank 1, the one after the catalytic converter, changes its reading more slowly than the engine computer expects.
- It's safe to drive on most cars, because the downstream sensor checks the converter rather than controlling the fuel mixture.
- An aging sensor is the usual answer, but exhaust leaks, wiring and software fixes should be ruled out before paying $150–$400 for a new one.
- Nissan, Infiniti, Mitsubishi and Alfa Romeo have bulletins that fix P0139 with an engine computer update on some models.
- 3 owner complaints filed with NHTSA mention P0139, and 25 manufacturer bulletins from 10 makes name it.
What P0139 means
The oxygen sensor behind your catalytic converter is reacting too slowly. Scanners call it “O2 Sensor Circuit Slow Response (Bank 1 Sensor 2).” Bank 1 is the side of the engine with cylinder 1 (a four-cylinder has only bank 1), and sensor 2 is the downstream sensor after the converter. The computer tests it at specific moments, often when you lift off the gas and fuel is cut, so the exhaust suddenly goes lean. A healthy sensor drops from high to low voltage quickly. If it takes longer than the computer allows, it sets P0139.
Is it safe to drive?
Yes. On most cars the downstream sensor only watches the converter; it doesn’t decide how much fuel the engine gets, so the car drives normally. Book a repair in the next few weeks, because a lit check engine light fails emissions inspections in most states and hides any new code.
The answer changes if the engine runs rough, smells of fuel, or the light starts flashing. Then something upstream is wrong, and unburned fuel can overheat the catalytic converter. Get it checked soon.
Causes, ranked
| Cause | How you confirm it | Typical cost (US independent shop, 2026) |
|---|---|---|
| Aged or contaminated sensor | High miles on the original sensor; lazy drop to low voltage on a decel test; oily or chalky deposits on the tip | $150–$400 |
| Engine computer software (some makes) | A bulletin for your exact model and year names P0139 | $100–$200, often $0 under warranty or a bulletin |
| Exhaust leak ahead of the sensor | Ticking or puffing at a joint, soot at the converter flange or flex pipe; smoke test | $150–$600 |
| Damaged wiring or corroded connector | Wire melted on the exhaust, chafed or corroded; signal changes when the harness is moved | $100–$250 |
| Oil or coolant getting into the exhaust | Oil burning, coolant loss, or an oil leak dripping onto the exhaust; fouled tip | $150–$400 for the sensor, plus fixing the source |
Trucks and sensors rusted into the pipe land at the top of these ranges.
What to check first
- Check bulletins before buying a sensor. This code has a clear software pattern. Mitsubishi reprograms the ECM for false check engine lights with P0139 on the 2014–2016 Outlander. Nissan and Infiniti have bulletins, several of them ECM reprograms, for the 2013–2015 Armada, 2012–2014 EX35, 2015 QX60, 2015 Altima and 2015 Juke, and the Armada bulletin says not to replace the O2 sensors on qualifying cars. Alfa Romeo has PCM flashes for the 2017–2018 Giulia and 2017 4C Spider, and Ford has a bulletin for 2023–2024 F-Super Duty 6.7L chassis cabs.
- Read every stored code. P0139 with the heater code P0141 points at the sensor. P0139 with P0159, the bank 2 twin, often means both are aging or a software issue.
- Check for exhaust leaks between the engine and the sensor, including converter flanges and the flex pipe.
- Inspect the harness under the car for heat damage, chafing, or water and corrosion in the plug.
- Watch a decel test on a scan tool. A shop snaps the throttle and lets off, then times how fast the downstream sensor drops. A slow drop with good wiring and no leak means the sensor.
- Clear it and drive. After a repair or update, clear the code and drive a few days of mixed trips so the monitor can rerun.
What it costs
The cheap outcome is a software update, often free under a bulletin or warranty, or a wiring repair at $100–$250. The common outcome is a new downstream sensor at $150–$400. The expensive outcome is an exhaust leak at a flange or flex pipe, $150–$600. Diagnosis first, usually $100–$180, is worth it here because a bulletin can make the sensor unnecessary. Only 3 owner complaints filed with NHTSA mention P0139, each on a different model. Our O2 sensor guide explains how shops test these sensors.
Related codes
P0139 often appears with P0159, the same fault on bank 2, and with P0136, P0137, P0138 or P0141 from the same sensor. If P0420 comes up too, fix P0139 first, because the converter test depends on this sensor. If the car feels fine, see check engine light on but the car runs fine.
What owners report to NHTSA
3 complaints filed with NHTSA mention P0139, 2 of them since 2015, across 3 different models. The component owners most often filed it under: engine.
Manufacturer service bulletins that name P0139
25 technical service bulletins and manufacturer communications filed with NHTSA mention P0139, from 10 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:
- FORD F-250 SD (2023, 2024), bulletin 10251461: Some 2023-2024 F-Super Duty chassis cab vehicles built on or before 02-Jan-2024 equipped with a 6.7L engine may exhibit an illuminated MIL with diagnostic trouble codes (DTC) P0133, P0139, and/or P0145 stored in the powe…
- ALFA ROMEO GIULIA (2017, 2018), bulletin 10170388: Flash: Powertrain Control Module (PCM) Updates This bulletin involves reprogramming the Engine Control Module (ECM), also known as a Powertrain Control Module (PCM) with the latest available software. Customers may exper…
- ALFA ROMEO 4C SPIDER (2017), bulletin 10219880: Flash: Powertrain Diagnostic and System Improvements This bulletin involves reprogramming the Engine Control Module (ECM), also known as a Powertrain Control Module (PCM) with the latest available software. Customers may…
- MITSUBISHI OUTLANDER (2014, 2015, 2016), bulletin 10091904: THIS TSB INSTRUCTS DEALERS TO REPROGRAM THE ECM WITH NEW SOFTWARE THAT WILL ADDRESS FALSE MIL ILLUMINATION WITH DTC P0139 OR P0171.
- LAND ROVER RANGE ROVER (2016), bulletin 10105881: "ISSUE: THE CUSTOMER MAY OBSERVE A MIL IN THE INSTRUMENT CLUSTER RELATING TO THE ENGINE SYSTEM (CHECK ENGINE LIGHT) AND ONE OR MORE CODES STORED IN THE PCM RELATING TO HEATED OXYGEN SENSOR (HO2S) IN THE EXHAUST SYSTEM. T…
- NISSAN JUKE (2015), bulletin 10192798: IF YOU CONFIRM One or more of the following DTCs has set: * P0037 - HO2 HTR (B1) (Heated oxygen sensor 2 heater control circuit low) * P0038 - HO2 HTR (B1) (Heated oxygen sensor 2 heater control circuit high) * P0137 - H…
Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.
Frequently asked questions
Can I drive with a P0139 code?
Yes, on most cars. The downstream sensor mainly checks the catalytic converter, so the engine runs the same. Fix it before your next emissions test, because the lit light fails the inspection.
What does slow response mean on a downstream sensor?
The computer tests the sensor during certain conditions, often when you lift off the gas and the exhaust goes lean. A healthy sensor drops quickly; an aged or contaminated one takes too long, and the computer sets P0139.
Will a new O2 sensor fix P0139?
On a high-mileage original sensor, often yes. But some Nissan and Infiniti bulletins tell technicians not to replace the O2 sensors for this code on qualifying cars and to reprogram the ECM instead, so ask about bulletins first.
Is P0139 a bad catalytic converter?
Not usually. A tired converter normally sets P0420. P0139 is about how fast the sensor reacts, though a slow sensor can make the converter test unreliable until it's fixed.