OBD-II Code P0139 — What It Means & How to Fix It

What Does This Error Mean?

P0139 means the Bank 1 Sensor 2 oxygen sensor is responding too slowly to changes in exhaust gases. A healthy downstream O2 sensor should react to exhaust chemistry within a certain timeframe. When the sensor is lazy or sluggish, the ECM detects the slow response and stores P0139. The most common cause is a worn-out oxygen sensor that has aged beyond its useful life. You may not notice drivability problems, but your emissions system is compromised.

Affected Models

All vehicles 1996+
Common in Honda Pilot
Common in Toyota Sienna
Common in Nissan Pathfinder
Common in Chevrolet Traverse

Common Causes

Aging or worn-out downstream oxygen sensor with a degraded sensing element
Contaminated sensor from oil consumption, coolant leak, or silicon-based sealant
Heat damage to the sensor body or wiring from extended exposure near the catalytic converter
Partially clogged heater element causing the sensor to run cooler than optimal
Failing catalytic converter generating abnormal exhaust chemistry that masks proper response

How to Fix It

Connect an OBD-II scan tool and look at the live data for Bank 1 Sensor 2. Watch the sensor voltage while the engine is at operating temperature. A healthy sensor should show a relatively stable voltage around 0.6-0.8V, with occasional slow fluctuations.

Unlike the upstream sensor which switches rapidly between rich and lean, the downstream sensor is normally more stable. But it should still respond when exhaust conditions change noticeably.
Check the sensor's heater function. P0139 can be caused by a partially failed heater element that keeps the sensor cooler than ideal, making it sluggish. Check for a companion heater code like P0036 or P0037.

A cold sensor reacts slowly to exhaust chemistry. If the heater has partially failed, the sensor body never reaches full operating temperature and appears lazy to the ECM.
Inspect the sensor and its wiring visually. Look for carbon deposits, oily fouling, or burned insulation on the harness. An oily sensor tip indicates engine oil consumption — burning oil contaminates the sensor over time.

If the engine burns oil (blue exhaust smoke, frequent oil top-offs needed), the contamination will recur after sensor replacement. Have the oil consumption issue investigated as well.
Remove the sensor and inspect the tip. A healthy sensor tip should be light gray or tan. A sensor covered in black carbon soot, white silicone deposits, or oily residue is contaminated and will not respond properly.

Silicone contamination comes from using non-oxygen-sensor-safe RTV sealant on the engine. Only use sealant labeled safe for O2 sensors around intake and exhaust areas.
Replace the Bank 1 Sensor 2 oxygen sensor. This is almost always the correct repair for P0139 once heater and contamination issues are ruled out. Use an OEM or reputable brand sensor. Clear the code and drive to verify.

Downstream oxygen sensors typically last 60,000-100,000 miles. On a high-mileage vehicle, this code is often the sensor simply reaching the end of its service life.

When to Call a Professional

P0139 almost always points to a sensor that simply needs replacement. A downstream O2 sensor typically costs $80-$200 installed at an independent shop. If you also have P0420 (catalyst efficiency) alongside this code, a mechanic should evaluate the catalytic converter. Catalytic converter replacement costs $500-$2,500 depending on the vehicle and whether OEM or aftermarket. Diagnosis is straightforward — a live data scan instantly shows slow sensor response.

Frequently Asked Questions

What does slow response mean for an oxygen sensor?

Think of the oxygen sensor like a thermometer. A fast thermometer shows temperature changes instantly. A slow thermometer takes a while to catch up to the actual temperature. A slow O2 sensor takes too long to respond when exhaust chemistry changes. The ECM measures how quickly the sensor reacts and flags it if the response time is too slow.

How is P0139 different from P0136?

P0136 means the sensor circuit itself has a problem — voltage is out of range entirely. P0139 means the sensor is electrically alive but responding too slowly. P0136 is more likely to be a wiring or connection issue. P0139 usually means the sensor has aged out and needs replacement. Both affect the same Bank 1 Sensor 2 downstream oxygen sensor.

Can P0139 cause my car to fail emissions?

Yes — the check engine light will be on, which fails a visual emissions inspection. P0139 also disables proper catalytic converter monitoring. If the converter is also failing, P0139 may be hiding that problem. Fix this code before an emissions test to ensure all systems are working and monitored correctly.