P0140: O2 Sensor Circuit No Activity Detected Bank 1 Sensor 2

What does P0140 mean?

The oxygen sensor after the catalytic converter on bank 1 (sensor 2) is a switching sensor (about 0.1–0.9 V) used for converter monitoring and fine mixture trim. P0140 means the sensor produces no voltage of its own and the control module only sees its own reference (bias) voltage. GM's V6 sets it when the signal sits at 401–548 mV or the sensor's internal resistance exceeds 40 kΩ when it should be hot; newer GM systems apply 1900 mV and read above 1700 mV as an open circuit. Two trips. Ford reports 'no activity' under P0136 instead. A cold sensor with a failed heater can also cause it. When it fails, the control module stops the downstream trim and cannot evaluate the converter; there is no limp mode.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on (usually after two drive cycles)
• Catalyst monitor readiness not set
• Live data: sensor 2 stuck at about 450 mV (or 1.9 V)

Common causes

• High Signal or ground wire open, connector off or corroded
• Medium Sensor heater failed, sensor never becomes active
• Medium Sensor worn out (no longer produces voltage)
• Low Control module input faulty

Likelihood is a general estimate across vehicles; your make and model may differ.