P0140: O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
The downstream oxygen sensor on bank 1 is not showing any signal activity after the engine has warmed up.
What P0140 means
P0140 means the engine computer saw no activity from the downstream oxygen sensor on bank 1 after the engine and exhaust warmed up. RepairPal describes this sensor as the catalyst monitor, judging the converter ability to clean the exhaust. With no signal, the computer cannot run that test, so the car may fail an emissions inspection, and RepairPal notes NOx emissions may also rise. Usually there are no driveability symptoms. The most common causes are a dead sensor, a heater that has failed so the sensor stays cold, and an open wire or bad connector. Downstream connectors sit under the car where water and salt reach them, so corrosion is common. RepairPal warns that sensors are sometimes replaced when the real problem was the converter or the wiring. Shops check heater resistance, look for the bias voltage at the connector and watch the signal on a long drive. The fix is usually a sensor or a wiring repair.
Common symptoms
- Check engine light on
- Usually no noticeable symptoms
- Failed emissions test
- Higher NOx emissions in some cases
- May show up with P0141
Most common causes
- Dead downstream oxygen sensor
- Failed sensor heater
- Open signal or ground wire
- Corroded or unplugged connector
- Catalytic converter problem
What to check yourself first
Check the downstream sensor connector, which often sits on the transmission or floor pan and collects dirt and water. Look for damaged wires. If P0141 is also set, the heater failure may be the root cause.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace oxygen sensor | $440-$523 | When the sensor stays flat with good wiring and a working heater. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | RepairPal lists faulty wiring and connections as a common misdiagnosis. |
| Check engine light diagnosis at a shop | $125-$183 | Checks whether the converter or wiring is the real issue. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0141 O2 Sensor Heater Circuit (Bank 1 Sensor 2)
- P0136 O2 Sensor Circuit (Bank 1 Sensor 2)
- P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
- P0134 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)