# P0139: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) **Verdict:** Safe to drive Oxygen sensors - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $150-$400 - **Urgency:** Keep driving - **Most likely:** Aging, slow downstream O2 sensor - **DIY?:** Yes, moderate ## Quick answer P0139 means the downstream oxygen sensor on bank 1, after the catalytic converter, is reacting too slowly when the exhaust changes from rich to lean. An aging or contaminated sensor is the most common cause. The car usually drives normally, but the converter test cannot be trusted until it is fixed. ## What does P0139 mean? The downstream oxygen sensor on bank 1 sits after the catalytic converter. The computer uses it to judge how well the converter stores oxygen, and on many cars for small fuel corrections. To trust that judgment, the computer also has to trust that the sensor itself reacts quickly when the exhaust changes. P0139 sets when the downstream sensor takes too long to respond during the computer's self-test. On many cars the test runs during deceleration fuel cut: with your foot off the gas, the exhaust quickly fills with oxygen, and a healthy sensor drops from a high to a low reading within a short, maker-defined time. Some makers also check how fast it rises on the way back. If the change is too slow, usually on two trips, the code sets. Bank 1 is the side with cylinder 1, and sensor 2 is after the converter. Unlike P0137 or P0138, the voltage range is normal; it is the speed that is wrong. ## What are the symptoms of P0139? - Check engine light on - Car drives normally in most cases - Catalyst monitor not ready, or a failed emissions test - Sometimes appears with P0420 on high-mileage cars ## What causes P0139? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Aging downstream oxygen sensor** (most common). As an oxygen sensor ages, deposits build on its tip and the sensing element reacts more slowly. Downstream sensors often last a long time, but many high-mileage originals eventually become too sluggish for the deceleration test. A sensor that still reaches normal high and low voltages, just slowly, fits this cause. 2. **Sensor contaminated by oil, coolant or additives** (common). An engine that burns oil, leaks coolant into the combustion chamber or uses heavy fuel additives can coat the sensor tip and slow its response. Contamination can affect a fairly new sensor if the source is not fixed. Signs include oil use between changes, coolant loss, or white or blue exhaust smoke. 3. **Exhaust leak near the sensor** (less common). A small leak near the downstream sensor can dilute and blur the change from rich to lean exhaust, so the sensor seems slow. Rusted flanges, flex pipes and cracked converter shells are typical on older vehicles in road-salt states. Look for soot marks near joints, and listen for puffing on a cold start. 4. **Weak sensor heater or poor connection** (less common). A sensor that runs too cool reacts slowly. A heater that is weakening, or a high-resistance connection in the heater or signal circuit, can slow the response without setting a separate heater code at first. Corroded connectors under the car are common in snow-belt states. Checking heater resistance and connector condition helps rule this out. 5. **Engine computer software** (rare). On some vehicles, makers have revised the downstream sensor test with a software update after finding it too sensitive. This is uncommon and specific to certain models. A shop can check technical service bulletins for your vehicle before replacing a sensor, especially if a new sensor did not fix the code. ## How do you diagnose P0139? 1. Read all codes; oil-burning or coolant problems and P0420 change what to check first. 2. Warm the engine, hold about 2,500 RPM, then close the throttle; watch how quickly bank 1 sensor 2 falls from high to low during fuel cut. 3. Compare the response with the maker's specification or a known-good sensor, if available. 4. Inspect for exhaust leaks around the converter and sensor, and check the connector for corrosion. 5. Test the heater circuit resistance and check service bulletins before replacing the sensor. ## How do you fix P0139, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace the downstream oxygen sensor | $150-$400 | The usual fix for a slow sensor; use a direct-fit part for bank 1 sensor 2. | | Repair an exhaust leak near the sensor | $100-$400 | Needed when a leak is blurring the sensor reading. | | Fix oil burning or coolant entry | $100-$2,500 | Ranges from a PCV valve to gasket repairs; prevents the new sensor from being contaminated. | Full price breakdowns: [oxygen sensor replacement cost](/cost/oxygen-sensor-replacement). ## Is it safe to drive with P0139? > **Safe to drive.** Yes, it is safe to drive. P0139 rarely changes how the car runs, but it will keep the check engine light on and fail an emissions test until the sensor is fixed. ## Will P0139 fail an emissions test? P0139 keeps the check engine light on, which fails an OBD-II emissions test in most states. Because the downstream sensor is the computer's tool for checking the converter, the catalyst monitor cannot give a valid result while it is slow. After the repair and clearing the code, drive several days with steady cruising and some coasting before testing. ## After the repair: clearing P0139 Clear the code after the repair. The downstream sensor test usually runs during coasting with a warm engine, so include some long, gentle decelerations from highway speed in your next few drives. When the oxygen sensor and catalyst monitors show complete and no pending code returns, the repair worked. ## How to prevent P0139 - Fix oil burning and coolant leaks early, since both coat and slow oxygen sensors. - Repair exhaust leaks near the converter before they blur the downstream reading. - Avoid regular use of heavy fuel additives unless your owner manual recommends them. - Use a direct-fit sensor with the correct connector, rather than splicing in a universal one. ## Codes often seen with P0139 - [P0133](/codes/p0133): O2 Sensor Circuit Slow Response (Bank 1 Sensor 1) - [P0136](/codes/p0136): O2 Sensor Circuit (Bank 1 Sensor 2) - [P0137](/codes/p0137): O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) - [P0138](/codes/p0138): O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) - [P0141](/codes/p0141): O2 Sensor Heater Circuit (Bank 1 Sensor 2) - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) ## Frequently asked questions ### Can I drive with a P0139 code? Yes, P0139 is safe to drive with. The downstream sensor mainly monitors the catalytic converter, so the engine usually runs as before. The light will stay on and you will fail an emissions test, so plan to replace the sensor within a few weeks if your area requires testing. ### What is the difference between P0133 and P0139? Both mean an oxygen sensor on bank 1 responds too slowly. P0133 is the upstream sensor ahead of the converter, which controls fuel and can affect mileage. P0139 is the downstream sensor after the converter, which mainly checks the converter. P0139 usually has fewer noticeable symptoms. ### How much does it cost to fix P0139? Most P0139 repairs cost $150 to $400 at an independent shop, mainly for a new downstream oxygen sensor. An exhaust leak repair near the sensor runs about $100 to $400. If oil burning or a coolant leak is contaminating the sensor, that repair can cost much more. ### Can I replace the sensor myself for P0139? Often, yes. The downstream sensor is usually reachable from under the car behind the converter with an oxygen sensor socket. Support the car safely on stands, let the exhaust cool and use penetrating oil on rusted threads. Make sure the new part is for bank 1 sensor 2. ### Does P0139 mean my catalytic converter is failing? No, P0139 is about the downstream sensor reacting slowly, not the converter itself. The two can appear together on high-mileage cars, though. Fix the sensor first, drive several trips, and see whether P0420 returns before deciding the converter is bad. ### Why did P0139 come back after a new sensor? The new sensor may be contaminated by oil or coolant the engine is still burning, or an exhaust leak may be blurring the reading. A wrong or low-quality part can also be slow. Check for oil use, coolant loss and leaks, and ask the shop to look for service bulletins. ### How is the downstream sensor tested for slow response? Many cars test it during deceleration fuel cut. When you lift off the gas, fuel is cut and the exhaust quickly turns lean, so a healthy downstream sensor should drop from high to low within a short time. The computer times that drop, and a slow drop sets P0139. ### Does P0139 affect gas mileage? Usually very little. The upstream sensor handles most fuel control, and the downstream sensor only makes small corrections on many cars. If your mileage has dropped noticeably, look for another problem, such as an upstream sensor code or a lean or rich condition. ### How long do downstream oxygen sensors last? Downstream sensors often last longer than upstream ones because they run cooler and see cleaner exhaust. Many go well past 100,000 miles, but oil burning, coolant leaks and rough conditions shorten their life. There is no fixed interval, so replace one when testing shows it is slow or failed. ### Can a cleaner or additive fix P0139? It is unlikely. Additives cannot reliably remove deposits from an aged sensor tip, and some can add to the buildup. If the sensor is slow from age or contamination, replacement is the lasting fix, along with repairing any oil or coolant leak that caused it. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other oxygen sensors codes - [P0131 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0131) - [P0132 O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0132) - [P0133 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0133) - [P0134 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0134) - [P0135 O2 Sensor Heater Circuit (Bank 1 Sensor 1)](https://diagxcar.com/codes/p0135) - [P0136 O2 Sensor Circuit (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0136) - [P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0137) - [P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0138) - [P0140 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0140) - [P0141 O2 Sensor Heater Circuit (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0141) ## Symptoms - [Why is blue smoke coming from my exhaust?](https://diagxcar.com/symptoms/blue-smoke-from-exhaust) - [Why is white smoke coming from my exhaust?](https://diagxcar.com/symptoms/white-smoke-from-exhaust) ## Repair costs - [Oxygen sensor replacement cost](https://diagxcar.com/cost/oxygen-sensor-replacement): $150-$450 --- Source page: https://diagxcar.com/codes/p0139 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.