OBD codes / P0139
P0139
ModerateO2 Sensor Circuit Slow Response (Bank 1, Sensor 2)
There is no drivability penalty for driving with P0139 — the rear sensor is a monitor, not a controller. Treat it as an emissions-compliance item: fix it before an inspection, and before trusting any catalytic-converter diagnosis, since a lazy rear sensor skews that test.
What this code 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.
Symptoms you might notice
- Check-engine light and nothing else in most cases
- No drivability or fuel-economy change
- Failed emissions test
What it costs to fix
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 →
Frequently asked
How urgent is P0139?
Low. The car runs exactly the same with or without a healthy rear sensor. 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?
Yes — 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 be fine.
Seeing P0139 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0139 for my vehicleRelated trouble codes
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Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.