P0140 means O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2). The downstream oxygen sensor on bank 1 is not producing a usable signal; it stays near a fixed voltage. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0140 means
Oxygen sensors measure the oxygen left in the exhaust. Sensor 1 sits before the catalytic converter and is used for fuel control; sensor 2 sits after it and mainly monitors the converter. Most sensors have a heater so they start working soon after a cold start. A traditional zirconia sensor switches between about 0.1 volt (lean) and 0.9 volt (rich); many newer upstream sensors are wide-band air-fuel sensors that report differently, so check which type your engine uses.
P0140 symptoms
- Check engine light
- Usually no drivability change
Common causes of P0140
- Failed sensor
- Open signal wiring or unplugged connector
- Failed heater, so the sensor never warms up
How to diagnose P0140
- Check the connector
- Check heater resistance and power
- Watch sensor voltage after a full warm-up drive
Can you drive with P0140?
Usually yes for a while, but fix it to protect fuel economy and the catalytic converter.
P0140 tips
A dead heater keeps the sensor too cold to work; check the heater circuit before replacing the sensor.
Will P0140 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0140 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0137 | O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) |
| P0138 | O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) |
| P0141 | O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2) |
Guides that help
When it is more than a sensor
Oxygen sensor codes are fixed with a sensor, wiring, a fuse or an exhaust leak repair. An engine burning oil or coolant contaminates sensors quickly; if new sensors keep failing, check for blue or white exhaust smoke and a head gasket leak, and compare engines by code on our engines page if the engine is worn out.
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