P0139 code: O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)
P0139 means o2 Sensor Circuit Slow Response (Bank 1, Sensor 2). The code says where to look, not what failed — the fix depends on which cause your car actually has.
What P0139 means
P0139 means the downstream oxygen sensor on Bank 1 — behind the catalytic converter — reacted too slowly when the computer expected a quick change. The computer periodically tests the rear sensor, often during fuel cut on deceleration, when the exhaust suddenly goes lean; a healthy sensor should register that swing promptly. When it lags, the code sets. The usual cause is an aging sensor whose element has slowed down, with contamination (oil, coolant, silicone) and marginal wiring as runners-up; an exhaust leak near the sensor can also blur the reading. Because Sensor 2's role is monitoring the catalyst rather than running the engine, you will not feel this fault — but a lazy rear sensor makes the converter-efficiency check unreliable, which matters when diagnosing or clearing P0420-type problems.
P0139 symptoms
- Check-engine light and nothing else in most cases
- No drivability or fuel-economy change
- Failed emissions test
What it costs to fix P0139
Typical range: $100–$450 · about ~0.3–1 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| Downstream oxygen (O2) sensor | $30–$250 | ~0.3–1 hrs |
| Exhaust leak repair near the sensor | $20–$200 | ~0.5–2 hrs |
A slow rear sensor is nearly always replaced rather than repaired — typically a lower-cost O2 job thanks to easier access. A seized sensor or a rusted exhaust bung is the main thing that pushes the price up.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0139 for my exact vehicle →
How to fix P0139
Easy — basic tools, most owners can do this
- Check the Mode 06 results for the rear sensor
Many scan tools can display the on-board test results (Mode 06) for each oxygen sensor, including the downstream response-time test and its pass limits. A result far past its limit confirms a genuinely lazy sensor; one sitting right on the edge is worth rechecking after a few more drives before spending anything.
- Run a deceleration fuel-cut test on the road
With a passenger watching live data, get the engine fully warm, then coast in gear from highway speed with your foot completely off the pedal. When the injectors shut off, the Bank 1 Sensor 2 voltage should fall quickly toward the bottom of its range; a slow, sagging drop that takes seconds is exactly what this code describes.
- Rule out a leak at the converter outlet
Inspect the flange behind the catalytic converter and the sensor's own threaded boss for soot streaks or a puffing sound at cold start. Outside air drawn in right beside the sensor muddies the very transition the computer is trying to time.
- Check the old sensor's tip for poisoning
Remove the sensor and read its tip: dark oily soot, chalky white residue, or a gray glaze from silicone means something in the engine is poisoning the exhaust, and a new sensor will slow down the same way. A clean but aged tip on a high-mileage car simply means it wore out.
- Replace the downstream sensor, then judge the catalyst
Install a quality replacement with the car safely on stands, then drive normally until the oxygen-sensor and catalyst monitors report complete. Judge any P0420 stored alongside only after the new sensor has been through those monitors — a lazy rear sensor can fail a healthy converter's test.
DIY or shop? Slow-response diagnosis suits a home mechanic with a scan tool that shows Mode 06 or graphs live data, and the coast-down test needs only a safe road and a passenger to watch the screen. The rear sensor sits behind the converter where there's usually room to swing a wrench, so replacement is typically a short job. Pay a shop if the sensor is rusted solid, if the threads in the pipe are damaged, or if the old tip shows coolant or oil contamination — that engine problem is the bigger repair and needs proper testing.
Tools for the DIY steps: Scan tool with Mode 06 or graphing · Jack stands or ramps · O2 sensor socket · Penetrating oil · Thread chaser for the sensor boss
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0139.
Start a case with P0139 and your car. Redline weighs these causes against what your car shows, and names the check that would settle it before you buy parts.
Can I drive with P0139?
Redline can’t determine from a symptom or code alone whether your car is safe to drive.
P0139: common questions
How urgent is P0139?
Low. The rear sensor doesn't control the engine, so you may not notice any change. The reasons to fix it are passing emissions testing and restoring the computer's ability to monitor the catalytic converter accurately.
Why does a slow O2 sensor even matter downstream?
The computer compares upstream and downstream sensor behavior to grade the catalytic converter. If the rear sensor responds sluggishly, that comparison is garbage — it can flag a good converter as bad or pass a failing one.
Is P0139 the same problem as P0133?
Same fault type, different position: P0133 is slow response at the upstream sensor (which affects fuel control), P0139 is slow response at the downstream one (which only affects catalyst monitoring). The downstream version is the more benign of the two.
Can an exhaust leak cause P0139?
It can — a leak near the rear sensor lets pulses of outside air reach the sensor tip, muddying the clean lean-rich transitions the computer looks for during its response test. Check for leaks before replacing a sensor that might not be at fault.
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0139 from your phone. See our guide to the best OBD2 Bluetooth adapters. Run a repair shop? See Redline for shops.
Generic OBD-II reference. Manufacturer-specific behavior varies. Confirm with a scan tool and, for safety-related codes, a professional inspection. Cost ranges are reference ranges, not quotes.
Seeing P0139 on your car?
Every problem becomes a Case. Tell Redline what the car is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.