P0140: O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
P0140 means the engine computer is not seeing the rear oxygen sensor on Bank 1 switch the way it should. The most common fix is replacing a bad O2 sensor or repairing its wiring, which typically costs about $150 to $400 at an independent shop.
- Severity
- Moderate
- Can I drive?
- Drive with caution
- DIY?
- Possible with some experience
- Repair cost range
- $100 – $450
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 P0140 mean?
Bank 1 Sensor 2 is the oxygen sensor located after the catalytic converter on the engine side that contains cylinder #1. The computer watches this sensor to help check catalytic converter performance and keep emissions under control. If the sensor signal stays flat or inactive, the ECU sets P0140. This can happen because the sensor is worn out, the wiring is damaged, or the exhaust has a leak that affects the reading.
Common symptoms
- Check engine light on
- Usually no noticeable drive problem
- Possible bad gas mileage
- Failed emissions test
- Occasional rough running if another issue is present
Causes, fixes and costs
P0140: O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
General guide for most cars. Your car may differ.
The car will often still drive normally, but the emissions system cannot monitor the catalytic converter correctly. If you also notice a strong exhaust smell, loud exhaust noise, or worsening drivability, have it checked sooner.
Most likely causes
- 1
Faulty Bank 1 Sensor 2 oxygen sensorHigh
A worn-out rear O2 sensor is the most common reason for a flat or inactive signal, especially if the car has higher mileage and only the check engine light is on.
Quick check: Scan live data and watch the rear sensor voltage; if it stays nearly fixed while the engine is warmed up, the sensor is suspect.
- 2
Damaged wiring or connectorHigh
If the light came on after road debris, exhaust work, or water exposure, broken wires, corrosion, or a loose connector can stop the sensor signal.
Quick check: Inspect the harness near the exhaust for melted insulation, broken wires, corrosion, or a connector that is not fully seated.
- 3
Exhaust leak near the rear sensorMedium
An exhaust leak ahead of or near the sensor can let outside air in and confuse the reading, sometimes with a ticking or hissing exhaust sound.
Quick check: Look for soot marks, listen for leaks on a cold start, and check the exhaust for cracks or loose joints near Bank 1 Sensor 2.
- 4
ECM or heater circuit issueLow
If the sensor and wiring test good but the signal still does not move, the problem may be in the control circuit or the engine computer input.
Quick check: Verify heater power and ground, then test the circuit with a scan tool or multimeter before replacing parts.
Can you fix it yourself?
Possible with some experience
Difficulty: Moderate. Replacing the sensor is a fair DIY job for someone comfortable working under the car, but rusted threads and hot exhaust parts can make it harder. You will need the correct O2 sensor socket or wrench, basic hand tools, and possibly penetrating oil; if wiring is damaged, ask the shop to repair the harness properly.
Show the stepsHide the steps
- Let the exhaust cool completely
- Locate Bank 1 Sensor 2 behind the catalytic converter
- Inspect the connector and wiring first
- Unplug the sensor and check for corrosion or melted wires
- Remove the sensor with the correct O2 sensor socket
- Install the new sensor with the correct torque
- Reconnect the wiring and clear the code
- Drive the car until it reaches full operating temperature and recheck for codes
Estimated repair cost
$150 – $400
Replace Bank 1 Sensor 2 oxygen sensor: parts about $50-$180, labor about $100-$220
- Repair oxygen sensor wiring or connector$120 – $300
- Fix exhaust leak near rear sensor$100 – $450
- Diagnose ECM or circuit fault$120 – $250
Prices vary by vehicle, rust level, and labor rates in your area.
Frequently asked questions
Can I drive with P0140?
Usually yes, if the car runs normally and the check engine light is steady. Get it checked soon because the vehicle may not pass emissions testing and the problem can hide other issues.
Will P0140 hurt my catalytic converter?
Not usually by itself, because this code is mainly a sensor or circuit monitoring fault. But if the engine is also running rich or misfiring, that can damage the converter over time.
Can a bad rear O2 sensor cause rough idle?
Usually not. A rear sensor mostly monitors the catalyst; rough idle points more toward a fuel, ignition, or vacuum problem, though multiple codes can occur together.
Will clearing the code fix it?
No, not if the sensor, wiring, or exhaust problem is still present. The light will come back after the computer runs its test again.
Is P0140 an emissions failure?
Yes, in most states it can fail an OBD emissions inspection if the check engine light is on or readiness monitors are not complete. Fix the cause and drive long enough for the monitors to reset before retesting.
Related codes
- P0136 O2 Sensor Circuit (Bank 1 Sensor 2)
- P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
- P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
- P0139 O2 Sensor Circuit Slow OBDResponse (Bank 1 Sensor 2)
- P0141 O2 Sensor Heater Circuit (Bank 1 Sensor 2)
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)
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.