# P0140: O2 Sensor Circuit No Activity Detected (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:** $20-$400 - **Urgency:** Keep driving - **Most likely:** Dead downstream O2 sensor or open circuit - **DIY?:** Steps 1-3 ## Quick answer P0140 means the downstream oxygen sensor on bank 1, after the catalytic converter, is sending no useful signal: its reading sits at a fixed value after the engine has warmed up. A dead sensor, an open wire or unplugged connector, or a failed sensor heater is the usual cause. ## What does P0140 mean? The downstream oxygen sensor on bank 1 sits after the catalytic converter. Once hot, a narrow-band sensor here normally reads fairly steady at cruise and moves low when fuel is cut on deceleration and higher when the engine runs rich. The computer uses that signal to check the converter and, on many cars, to make small fuel corrections. If the circuit is open or the sensor is cold, the computer sees only its own bias voltage, often about 0.45 volt. P0140 sets when the downstream signal stays in a narrow band around that bias voltage for too long after warm-up, even through conditions that should move it, such as coasting or acceleration. That differs from P0137 and P0138, where the voltage is stuck low or high, and from P0139, where it moves but too slowly. It usually needs two trips. Bank 1 is the side with cylinder 1, and sensor 2 is after the converter. ## What are the symptoms of P0140? - Check engine light on - Car usually drives normally - Heater code such as P0141 may appear at the same time - Catalyst monitor not ready, or a failed emissions test ## What causes P0140? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Dead downstream oxygen sensor** (most common). An old or contaminated sensor can stop producing a signal entirely, so the computer reads only its bias voltage. This is most likely on high-mileage original sensors and on engines that burn oil. On a scan tool, the reading stays flat near the middle of the range through coasting and hard acceleration on a fully warm engine. 2. **Open wire or unplugged connector** (common). The downstream connector and harness run under the car, where they can be pulled apart by road debris, rusted clips or exhaust work. A connector left unplugged after a converter or muffler job is a classic cause. A broken signal or ground wire gives the same flat reading, so check the plug and wiring first. 3. **Failed sensor heater, fuse or heater wiring** (common). The downstream sensor runs cooler than the upstream one and relies on its heater to stay at working temperature, especially at idle and in cold weather. If the heater, its fuse or its power feed fails, the sensor may never become active. A P0141 heater code alongside P0140 strongly points to this. 4. **Exhaust leak near the sensor** (less common). A large leak close to the downstream sensor can cool it and flood it with outside air, so it struggles to produce a changing signal. Rusted flanges, cracked flex pipes and holed converter shells are common on older cars in road-salt states. Soot marks near the sensor and a puffing sound on cold starts are clues. 5. **Engine computer fault** (rare). The engine computer supplies the bias voltage and reads the downstream signal. An internal fault can make a working sensor appear inactive. This is rare and should be considered only after the sensor, heater and wiring all test good. A shop may also check for software updates or bulletins that apply to your vehicle. ## How do you diagnose P0140? 1. Read all codes; a heater code such as P0141 with P0140 points to a sensor that is not getting hot. 2. Warm the engine fully and graph bank 1 sensor 2 while coasting and accelerating; a flat reading near about 0.45 volt confirms no activity. 3. Check that the downstream connector is plugged in and locked, and inspect the harness under the car for breaks. 4. Measure heater resistance and check the heater fuse and power feed. 5. Check for exhaust leaks near the sensor, then replace the sensor if the circuit and heater are good. ## How do you fix P0140, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace the downstream oxygen sensor | $150-$400 | Fixes a dead sensor or one with a burned-out heater. | | Reconnect or repair wiring or fuse | $20-$250 | Ranges from plugging in a loose connector to repairing a broken wire. | | Repair an exhaust leak near the sensor | $100-$400 | Needed if a leak is cooling or confusing the sensor. | Full price breakdowns: [oxygen sensor replacement cost](/cost/oxygen-sensor-replacement), [catalytic converter replacement cost](/cost/catalytic-converter-replacement). ## Is it safe to drive with P0140? > **Safe to drive.** Yes, it is safe to drive. The downstream sensor has little effect on how the engine runs on most cars, so the main costs are a check engine light that stays on and a failed emissions test. ## Will P0140 fail an emissions test? P0140 keeps the check engine light on, which fails an OBD-II emissions test in most states. With no downstream signal, the computer cannot check the converter, so the catalyst monitor stays not ready. After the repair and clearing the code, drive several days, including steady cruising and some long coasts, until the monitors show complete. ## After the repair: clearing P0140 Clear the code after the repair. Once warm, bank 1 sensor 2 should move: low when you coast with your foot off the gas, higher under light acceleration, and fairly steady at cruise. After several trips, the oxygen sensor and catalyst monitors should read complete with no pending code, showing the fix worked. ## How to prevent P0140 - After converter or exhaust work, confirm the downstream sensor is plugged in and its harness clipped up. - Fix heater codes such as P0141 promptly, since a cold sensor often leads to P0140. - Keep connectors under the car free of mud and road salt, especially in winter. - Repair exhaust leaks near the converter before they cool or confuse the sensor. ## Codes often seen with P0140 - [P0141](/codes/p0141): O2 Sensor Heater Circuit (Bank 1 Sensor 2) - [P0136](/codes/p0136): O2 Sensor Circuit (Bank 1 Sensor 2) - [P0137](/codes/p0137): O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) - [P0139](/codes/p0139): O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) - [P0134](/codes/p0134): O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1) - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) ## Frequently asked questions ### Can I drive with a P0140 code? Yes, P0140 is safe to drive with. The downstream sensor mainly checks the catalytic converter, so the car usually runs normally. The check engine light will stay on and the catalyst monitor cannot finish, so you will fail an emissions test until it is fixed. ### What does no activity mean for the downstream sensor? No activity means the sensor signal is not changing at all. Instead of dropping when fuel is cut and rising when the engine runs rich, it stays fixed, usually near the computer bias voltage of about 0.45 volt. That usually means a dead sensor, an open circuit or a sensor that is not heating. ### How much does it cost to fix P0140? Most P0140 repairs cost $20 to $400 at an independent shop. Reconnecting a plug or replacing a fuse is cheap, and wiring repairs run up to about $250. A new downstream oxygen sensor usually costs $150 to $400, depending on the part and how rusted the old one is. ### Why do I have P0140 and P0141 together? P0141 means the downstream sensor heater is not working, and a sensor that stays too cool often shows no activity, which sets P0140. When the two appear together, the sensor itself is usually at fault, though a blown heater fuse or broken connector can cause both. Check the fuse and plug first. ### Why did P0140 appear after a new catalytic converter? The downstream sensor was probably left unplugged, not fully locked or damaged during the converter job. Its harness may also be stretched or touching the hot pipe. Check the connector and wiring first, then confirm the sensor was moved to the new converter and installed correctly. ### Can I fix P0140 myself? Often, yes. Check the connector and wiring under the car, test the heater fuse, and replace the sensor with an oxygen sensor socket if needed. Support the car safely on stands and let the exhaust cool. Rusted sensors can be stubborn, so use penetrating oil and patience. ### What is the difference between P0134 and P0140? Both mean an oxygen sensor on bank 1 shows no activity. P0134 is the upstream sensor ahead of the converter, which controls fuel, so it can hurt mileage. P0140 is the downstream sensor after the converter, which mainly checks the converter and seldom affects how the car runs. ### Does P0140 hurt the catalytic converter? Not directly. The converter keeps working; the computer just cannot check it while the downstream sensor is silent. On some cars, losing the downstream fuel correction makes fuel control slightly less precise. Fix it within a few weeks so the converter monitor can run again and pass inspection. ### Is the downstream sensor heater fuse easy to check? Usually, yes. Your owner manual or the fuse box lid shows which fuse feeds the oxygen sensor heaters, often shared with other engine parts. A blown fuse can set codes on several sensors at once. If a new fuse blows again, look for a shorted heater or chafed wire. ### Will P0140 make my car fail inspection? Yes, in most states with OBD-II testing. The check engine light being on is a failure, and the catalyst monitor will not be ready while the downstream sensor shows no activity. After the repair, drive several days of mixed trips so the monitors can finish before you test. ## 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) - [P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0139) - [P0141 O2 Sensor Heater Circuit (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0141) ## Repair costs - [Oxygen sensor replacement cost](https://diagxcar.com/cost/oxygen-sensor-replacement): $150-$450 - [Catalytic converter replacement cost](https://diagxcar.com/cost/catalytic-converter-replacement): $1,000-$3,000 --- Source page: https://diagxcar.com/codes/p0140 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.