P0139: O2 Sensor Circuit Slow Response (Bank 1, Sensor 2) (Meaning, Causes, Fixes)
What P0139 means
Sensor 2 is the downstream oxygen sensor, mounted after the catalytic converter on bank 1 (the side of the engine with cylinder 1). RepairPal defines P0139 as the sensor's up-and-down 'cross-counting' voltage activity being too slow, which turns on the check engine light. OBD-Codes.com describes it differently, saying the air-fuel ratio is not being adjusted by the oxygen sensor signal or the ECM as expected, or not as often as expected, once the engine is warmed up. OBD-Codes.com also calls it the rear oxygen sensor on the driver's side, but which side bank 1 is on depends on the engine, so go by where cylinder 1 is. Exact test criteria vary by manufacturer.
Severity: can I keep driving?
Usually OK to drive
Get it checked within a reasonable time. RepairPal rates P0139 'Moderate', meaning it isn't usually an immediate safety risk, and OBD-Codes.com says you likely won't notice any drivability problems. Left alone, though, it can cause an emissions test failure, and RepairPal says the engine may ping and produce extra NOx in some cases. If the check engine light is flashing, the engine runs very rough, or you smell fuel or see heavy smoke, stop driving and have it looked at, because those point to a bigger problem than this sensor. Exhaust parts get very hot, so let the car cool fully before touching the oxygen sensor.
Common symptoms
- Check engine light on
- Often no other noticeable symptoms (both sources say this)
- Failed emissions test (listed by RepairPal)
- Engine pinging and higher NOx emissions in some cases (listed by RepairPal)
Likely causes
Roughly most to least common, based on the sources below. Your vehicle may differ.
- Faulty, aging or contaminated bank 1 sensor 2 oxygen sensor (OBD-Codes.com says this is the most likely cause)
- Broken, frayed or shorted sensor wiring, or poor connections
- Exhaust leak, especially in front of the sensor
- Defective catalytic converter (listed by RepairPal as a common cause; not listed by OBD-Codes.com)
- Intake air leaks or a faulty MAF sensor (OBD-Codes.com suggests checking these)
Diagnostic steps you can do
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. Fix misfire, fuel trim or catalytic converter codes such as P0420 first.
- OBD-Codes.com suggests the simplest first step is to clear the code and see whether it comes back.
- With the exhaust cold, find the bank 1 sensor 2 oxygen sensor just after the catalytic converter and check its wiring and connector for damage, fraying, melting or corrosion.
- Look and listen for exhaust leaks ahead of the sensor, such as a ticking or puffing sound at a cold start, or soot around joints, flex pipes and gaskets.
- Check for intake air leaks and make sure the air filter box and intake hoses are sealed.
Typical fixes
- Replace the bank 1 sensor 2 oxygen sensor
- Repair damaged or shorted wiring or a poor connection
- Repair an exhaust leak in front of the sensor
- Replace the catalytic converter only if testing shows it is defective
- Fix any intake air leak or MAF sensor fault found
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
- OBD-Codes.com lists checking the frequency and amplitude of the oxygen sensor signal as an advanced test, which needs a scan tool with live data or a lab scope
- RepairPal warns of three common misdiagnoses: replacing the sensor when the real problem is a defective catalytic converter, faulty wiring or connections, or an exhaust leak in front of the sensor
- The sources disagree on the catalytic converter: RepairPal lists it as a common trigger, while OBD-Codes.com points mainly to the sensor, wiring and leaks. A shop can test the converter before you pay for one.
- The code returns after a new sensor, or P0420 or fuel and misfire codes are also present
Where to buy
Plain search links. Always match parts to your exact year, make, model and engine.
FAQs
Can I drive with P0139?
Usually, for a while. RepairPal rates it 'Moderate', and both sources say there may be no symptoms. Get it diagnosed within a reasonable time, because it can make you fail an emissions test.
Which oxygen sensor is bank 1 sensor 2?
It is the sensor after the catalytic converter on bank 1, the side of the engine that has cylinder 1. On engines with a single exhaust, there is only bank 1.
Will replacing the oxygen sensor fix P0139?
Often. OBD-Codes.com says you will likely end up replacing the sensor if the code returns after clearing. But RepairPal warns that sensors are often replaced when the real problem is the catalytic converter, the wiring, or an exhaust leak in front of the sensor, so check those too.
What's the difference between P0139 and P0133?
Both are slow-response codes. P0133 is for bank 1 sensor 1, before the catalytic converter, while P0139 is for bank 1 sensor 2, after it.
Sources
Last reviewed 2026-10-24. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.