P0139: O2 Sensor Circuit Slow OBDResponse (Bank 1 Sensor 2)
P0139 means the rear oxygen sensor on bank 1 is switching too slowly for the engine computer’s expectations. The most common fix is replacing a lazy O2 sensor, which usually costs about $150 to $350 at an independent shop.
- Severity
- Moderate
- Can I drive?
- Drive with caution
- DIY?
- Possible with some experience
- Repair cost range
- $100 – $500
Get the answer for your exact car
Add your car and a few details. DashClue asks up to 3 questions, then tells you the most likely cause and what it should cost.
FreeNo card neededAnswer in about 2 minutes
What does P0139 mean?
This code points to the downstream oxygen sensor, which sits after the catalytic converter on bank 1. That sensor helps the engine computer monitor exhaust oxygen and check how well the catalytic converter is working. When its signal reacts too slowly, the computer assumes the sensor may be worn out, contaminated, or not heating properly. Wiring problems or exhaust leaks near the sensor can also make the signal look sluggish.
Common symptoms
- check engine light on
- poor fuel economy
- rough idle
- failed emissions test
- no obvious drivability change
- may set alongside catalyst codes
Causes, fixes and costs
P0139: O2 Sensor Circuit Slow OBDResponse (Bank 1 Sensor 2)
General guide for most cars. Your car may differ.
The car usually keeps running, but fuel economy and emissions can suffer, and other emissions codes may appear. Stop driving if the engine starts misfiring badly, stalls, or the check engine light begins flashing.
Most likely causes
- 1
Worn or slow bank 1 sensor 2 O2 sensorHigh
Most P0139 cases come from a sensor that has aged and no longer changes voltage quickly. This is more likely if the light is on by itself and the car otherwise drives normally.
Quick check: Inspect sensor age and compare live data; the rear sensor should respond more slowly than the front, but not appear stuck or delayed for long periods.
- 2
Exhaust leak near the rear sensorMedium
A leak before or around the sensor can pull in outside air and confuse the reading, making the signal seem slow or unstable. This is more likely if you hear ticking, hissing, or smell exhaust near the underside of the car.
Quick check: Look for soot marks, loose joints, or damaged flex pipe areas near bank 1 sensor 2.
- 3
Damaged wiring or connectorMedium
Heat from the exhaust can melt wires or loosen the connector, which can slow the signal or interrupt sensor heater operation. This is more likely if the code comes and goes or appears after hitting bumps or driving in wet weather.
Quick check: Inspect the harness for rubbed-through insulation, corrosion, or a loose plug at the sensor.
- 4
Sensor heater circuit problemLow
If the sensor does not heat up quickly enough, it may respond slowly after cold start and trigger the code. This is more likely if the problem shows up soon after starting the car and other O2 heater codes are also stored.
Quick check: Check for related heater codes and verify power and ground at the sensor heater circuit.
Can you fix it yourself?
Possible with some experience
Difficulty: Moderate. A handy owner can inspect the sensor wiring and exhaust for visible problems, and sometimes replace the sensor with basic tools. You’ll need the correct bank 1 sensor 2 replacement, an O2 sensor socket, and possibly penetrating oil for a rusted sensor.
Show the stepsHide the steps
- Verify the code and check for related O2 or heater codes
- Inspect the exhaust and sensor wiring for damage or leaks
- Clear the code and see if it returns
- If no wiring or exhaust issue is found, replace the bank 1 sensor 2 O2 sensor
- Use anti-seize only if the new sensor instructions allow it
- Clear codes and road test the car
- Recheck live data to confirm the sensor now responds normally
Estimated repair cost
$150 – $350
Bank 1 sensor 2 oxygen sensor replacement: parts ~$50-$150, labor ~$100-$200
- O2 sensor wiring repair$100 – $250
- Exhaust leak repair near rear sensor$120 – $500
- Diagnostic test and live-data check$100 – $180
Prices vary by vehicle, rust, and local labor rates.
Frequently asked questions
Can I drive with P0139?
Usually yes for short trips, but you should get it checked soon. The car may run normally, but it can waste fuel and fail an emissions test.
Will P0139 fail emissions testing?
Yes, in most states it can fail because the check engine light is on and the monitor may not be ready. The code also suggests the emissions system is not being monitored correctly.
Do I need to replace the catalytic converter for P0139?
Usually no. P0139 is more often an oxygen sensor, wiring, or exhaust leak problem than a bad catalytic converter.
Can I clean the oxygen sensor to fix P0139?
Sometimes contamination can be part of the problem, but cleaning is rarely a reliable fix. Most slow-response sensor problems end up needing replacement.
Is bank 1 sensor 2 the front or rear O2 sensor?
It is the rear sensor on bank 1, located after the catalytic converter. Bank 1 means the side of the engine with cylinder number 1.
Related codes
- P0133 O2 Sensor Circuit Slow OBDResponse (Bank 1 Sensor 1)
- P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
- P0140 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
- P0141 O2 Sensor Heater Circuit (Bank 1 Sensor 2)
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)
- P0430 Catalyst System Efficiency Below Threshold (Bank 2)
Related problems
This guide covers most cars and is general advice, not an inspection. Prices are typical US independent-shop ranges and vary by region, car and shop. If your brakes, steering or a warning light feel unsafe, stop driving and contact a mechanic.