P0139 Code: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
The oxygen sensor after the converter on bank 1 takes too long to react when fuel is cut on deceleration.
At a glance
Severity: Minor
- Meaning
- O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $40–$300
- DIY difficulty
- Easy
The downstream sensor normally sits in the rich half of its range behind a working converter, so the PCM can't judge its speed during ordinary cruising. Instead, most engines test Bank 1 Sensor 2 during a deceleration fuel cut: the injectors switch off, a flood of oxygen passes through the converter, and the PCM times how long the rear sensor needs to fall from its rich level to below a lean threshold, often somewhere around 0.2 V. When that drop takes longer than the calibrated limit, or the sensor recovers too slowly once fuel returns, P0139 is set. The usual reason is an aged sensor whose element has lost its sensitivity after many years under the car. Oil ash, coolant deposits and a weak heater can make it lazy sooner, and because the test needs coasting in gear, the code may take a while to reappear after a repair or to confirm a fix.
Can I keep driving? No effect on how the car drives and no immediate risk. Replace the sensor at your convenience, but before an emissions inspection, since the catalyst monitor depends on it.
How to read P0139
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 39 Specific fault
Symptoms
- Check engine light with no other noticeable symptoms
- Catalyst monitor fails to complete or reports a failure
- Rear sensor drifts down slowly during coast-downs instead of dropping sharply
- Code often returns only after highway driving with long decelerations
- Sometimes accompanied by P0420 on high-mileage vehicles
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Downstream sensor aged after many years of heat cycling
- Element coated with oil ash or coolant residue carried through the converter
- Heater weak or intermittent, so the sensor runs cool in the post-converter exhaust
- Exhaust leak near the sensor smearing the signal
- Low-quality or incorrect replacement sensor
- Corroded connector or poor sensor ground
- PCM calibration issue addressed by a software update
Diagnosis step by step
Check the rear sensor test in Mode $06
Look up the Bank 1 Sensor 2 results in Mode $06. Many vehicles show a rich-to-lean response time or a similar value with its limit, which tells you how far the sensor is from passing and confirms that the rear sensor, not the converter test, is failing.
Tools: OBD-II scanner
Watch a coast-down on the graph
Warm the engine fully, drive at about 50 mph and lift off completely in gear for five seconds; don't press the clutch or shift to neutral, because that cancels the fuel cut. A healthy rear sensor falls from roughly 0.7 V to below 0.2 V within about a second after the converter's oxygen store fills. A sensor that slides down over several seconds is slow.
Tools: OBD-II scanner
Verify the heater
Measure heater resistance at the sensor connector and compare with the spec, and check that the heater receives battery voltage with the engine running. A downstream sensor lives in cooler exhaust than the front one and depends heavily on its heater to stay responsive.
Tools: Multimeter
Look for a contamination source
Rear sensors are poisoned more slowly than front ones, but they still collect oil ash and coolant residue. Blue smoke, oil consumption between changes or unexplained coolant loss should be investigated, or the new sensor will age just as quickly.
Tools: Flashlight
Check for a leak near the sensor
Inspect the flange and pipe just ahead of the sensor for soot and rust holes. Outside air entering there disturbs the oxygen reading during the test and can make a good sensor look slow.
Tools: Flashlight
Replace and drive to confirm
Install a quality direct-fit sensor at about 30 lb-ft. Then drive with several long coast-downs from highway speed, for example on exit ramps, so the PCM can repeat the rear sensor test and set the monitor to complete.
Tools: O2 sensor socket, Torque wrench
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace downstream O2 sensor yourself (part) | Yes | $40–$130 |
| Replace downstream O2 sensor at a shop | No | $120–$280 |
| Repair exhaust leak ahead of the rear sensor | No | $80–$300 |
| PCM software update at a dealer | No | $100–$200 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
A lit check engine light for P0139 fails the OBD-II emissions test. Because the rear sensor test runs only during coast-downs, plan some highway driving after the repair so the catalyst and oxygen sensor monitors show ready.
Frequently asked questions
Is P0139 serious?
Not for the engine. The rear sensor doesn't control fueling on most vehicles, so the car drives normally. The real problem is that the catalyst monitor can't be trusted, which blocks an emissions pass.
Why does the PCM only test this sensor while I'm coasting?
Behind a working converter the rear sensor rarely moves, so there is nothing to time during normal driving. Cutting fuel on deceleration creates a sudden, predictable jump in oxygen that the PCM can measure the sensor's reaction against.
Does P0139 mean I need a new catalytic converter?
No. The converter's oxygen storage affects when the rear sensor starts to drop, but P0139 is about how fast the sensor itself responds. Replace the sensor first; only if P0420 appears afterwards should the converter be suspected.
Is a cheap universal sensor good enough?
Sometimes, but slow-response codes are exactly where low-quality sensors fall short. A direct-fit sensor of OE quality has the correct response characteristics and the factory connector.