P0140 — O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 2)
Low severityWhat it means
P0140 is stored when the engine control unit sees a fault involving the downstream oxygen sensor on bank 1 (sensor 2). The bank 1 downstream oxygen sensor sits after the catalytic converter and is used mainly to monitor how well the converter is working. “No activity” means the oxygen-sensor signal is flatlined, so the control unit cannot use it for fuel control at all.
Common symptoms
- A signal that stays flat instead of switching
- Often only a check engine light
- A possible failed emissions test
- Rarely any driveability change
Likely causes
- Dead sensor or failed heater element
- Open wiring or lost power/ground to the sensor
- Aged downstream oxygen sensor
- An exhaust leak after the converter
What to check first
- Check sensor heater power and ground, then the signal wiring
- Compare the upstream and downstream signals in live data
- Inspect the rear exhaust and the sensor wiring
Can you keep driving?
Generally safe to drive; repair to restore catalyst monitoring and pass emissions.
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