P0139: The rear oxygen sensor on bank 1 reacts too slowly to changes in the exhaust
O2 sensor circuit slow response (Bank 1 Sensor 2)
Severity: Monitor
A slow downstream oxygen sensor affects emissions and fuel economy rather than safety; TroubleCodes.net lists stumbling and rough idle as possible effects, so book a check when convenient.
P0139 in short
P0139 means the oxygen sensor on bank 1, sensor 2 is slow to respond to changes in the exhaust. A fouled or failed sensor, or damaged or corroded wiring and connectors, are typical causes. Fix any misfire, vacuum leak or exhaust leak first, since the engine must run correctly before the sensor can be judged.
The oxygen sensor reports how much oxygen is in the exhaust. Sensor 2 on bank 1 is the second sensor on that bank, normally the one after the catalytic converter. P0139 sets when it reacts to exhaust changes too slowly.
TroubleCodes.net lists damaged, burnt, shorted, disconnected or corroded wiring and connectors, open circuits, blown fuses, defective or fouled sensors, and rarely the computer. It says oil fouling is common on some European vehicles.
It stresses that the engine must be in good running condition first, with no misfire or vacuum or exhaust leaks, and suggests watching live sensor data and checking that the sensor has reached operating temperature.
Most likely causes
These are general causes for this code across vehicles. None is specific to one make or model.
Cause 1: Fouled, contaminated or failed oxygen sensor
Common general cause- Why it ranks here
- Defective and fouled sensors are named, and slow response describes a sensor that is aging or coated.
- What would change this
- A sensor with deposits or a sluggish live-data trace raises it.
Cause 2: Damaged, corroded or disconnected wiring or connector, open circuit or blown fuse
Common general cause- Why it ranks here
- TroubleCodes.net lists these before the sensor itself in its causes.
- What would change this
- A burnt or corroded wire, a loose connector or a blown heater fuse raises it.
Cause 3: Misfire, vacuum leak or exhaust leak affecting the readings
Possible — needs testing- Why it ranks here
- The page says the engine must run correctly, with no misfire or leaks, before diagnosing the sensor.
- What would change this
- Other misfire or lean codes, or a found leak, raise it.
Cause 4: Engine computer failure
Possible — needs testing- Why it ranks here
- Listed as rare.
- What would change this
- Consider it only after everything else tests good.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Record all stored codes and make sure there is no misfire, vacuum leak or exhaust leak.
IfMisfire or fuel-trim codes are also stored
It meansFix those first and recheck.
Points to: Misfire, vacuum leak or exhaust leak affecting the readings
IfNo other faults found
It meansInspect the wiring.
Step 2: Visually inspect the sensor wiring and connector for burns, damage, disconnection or corrosion, and check the related fuse.
IfDamaged or corroded wiring, a loose connector or a blown fuse
It meansRepair it.
Points to: Damaged, corroded or disconnected wiring or connector, open circuit or blown fuse
IfEverything looks sound
It meansWatch live data.
Step 3: With a scan tool, watch the sensor reading once the sensor is hot (TroubleCodes.net gives near 600 °F) and check its response.
IfThe sensor responds slowly or is coated with deposits
It meansThe sensor is at fault.
IfThe sensor responds normally
It meansRecheck the wiring for an intermittent fault.
Points to: Damaged, corroded or disconnected wiring or connector, open circuit or blown fuse
Symptoms
- Check engine light
- Reduced fuel economy
- Hesitation or stumbling on acceleration
- Rough idle
Compare with: Rough idle with the check engine light on
How to fix
Step 1: Check that the oxygen sensor connector is plugged in and the wiring is not burnt or chafed.
Parts and cost context
- Oxygen (O2) sensor:Fix misfires and leaks and check the wiring before replacing the sensor. Labour varies; get a quote.
Labor varies by vehicle and shop; get a quote before you approve a repair.
Compare used part prices on PartPriceGuideOn your vehicle
We don't have reviewed vehicle-specific evidence for P0139 yet. The causes and tests above apply across vehicles.
Related codes
No curated relationships for P0139 yet.
P0139 questions
Does sensor 2 mean the sensor before or after the catalytic converter?
Sensor 2 is normally the downstream sensor, after the catalytic converter. Check your vehicle's service information for which side of the engine is bank 1.
Sources and review
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error