P0140Powertrain
Powertrain · SAE
O2 Sensor Bank 1 Sensor 2 – No Activity
Meaning
The downstream oxygen sensor on Bank 1 shows no voltage variation — the signal is fixed or absent. The ECU cannot detect any switching in the downstream signal within the designated monitoring window.
Common causes
- Defective downstream O2 sensor (inactive sensing element or internal open circuit)
- Broken heater circuit preventing the sensor from reaching operating temperature
- Signal wire break or disconnection (open circuit)
- Signal wire short to ground fixing voltage at 0 V
- Corroded connector or loose terminals
- Severe sensor poisoning eliminating all sensitivity
Symptoms
- MIL (Check Engine) warning light on
- OBD-II catalyst monitor incomplete or not ready
- No noticeable driveability symptom in most cases
- Possible failure of emissions test
- Possible companion heater fault codes (P0141)
Severity
Moderata: The vehicle is driveable but the OBD-II system cannot evaluate the catalytic converter; the fault must be resolved to maintain emissions compliance.
What to do
- Read all DTCs and check sensor B1S2 live data: signal should oscillate or settle at ~0.6–0.7 V with the catalyst warm.
- Measure heater supply voltage and heater element resistance; verify correct heater ground.
- Check signal wire continuity from sensor to ECU and confirm absence of shorts to ground or to supply.
- Inspect the sensor connector for corrosion, bent terminals, or moisture ingress.
- Replace the downstream O2 sensor B1S2 if circuit electrical tests are within spec but the sensor remains inactive.