What does it mean?
OBD II fault code P0140 is defined as “ Oxygen (0 2 ) Sensor Circuit – No Activity Detected (Bank 1 Sensor 2)”, and is set when the PCM (Powertrain Control Module) does not detect a signal voltage from the oxygen sensor. On engines with two cylinder heads, “Bank 1” refers to the bank of cylinders that contains cylinder #1, and “Sensor 2” refers to the oxygen sensor located after (downstream) of the catalytic converter.
Note: While there may be a small time lag difference between #1 & 2 sensors, and #1 & 3 sensors where an additional sensor is fitted upstream of the converter, the difference will not be noticeable to anyone who is not an expert in interpreting waveforms.
The only important thing to remember here is that the signal voltages of all oxygen sensors change when the throttle setting changes, so “No Activity” means the same thing; there is no signal voltage, or the signal voltage does not change in response to a change in throttle setting, regardless of where an oxygen sensor is located. Oxygen sensors play a critical role in the fuel, ignition timing, and valve timing strategies calculated by the PCM.
The PCM collects information from various sensors around the engine to calculate the most suitable fuel/air ratio, ignition timing setting, as well as VVT/VCS settings to ensure that the engine delivers the most power with the least amount fuel used at any given engine speed or load. On most applications, the PCM provides a baseline signal voltage of around 450 millivolts when the sensor is cold.
As the heater control circuit warms up the sensor element to bring the sensor into operation sooner, the resistance (which is generally about 8 Ohms) starts to drop, which allows the sensor to generate signal voltages that are based on the amount of oxygen in the exhaust stream.
If the PCM detects that it takes longer than about 60 seconds or so for the sensor element to heat up, or that the signal voltage does not move outside of the 390-, to 490 millivolt range, it will deem the oxygen sensor to be inactive, set code P0140, and trigger the CHECK ENGINE light. Note however that the values given here are guidelines only; consult the manual for the vehicle being worked to get the exact values for that vehicle.
NOTE: Some Japanese vehicles, such as Toyota, do not use conventional oxygen sensors. Instead, these applications use AIR/FUEL RATIO sensors that measure the ratio between fuel and air in the exhaust stream. This type of sensor is NOT interchangeable with conventional oxygen sensors; consult the relevant manual for the correct diagnostic, testing, and repair procedures that pertain to this type of sensor.
The image below illustrates the differences between conventional oxygen sensors and AIR/FUEL RATIO sensors.