There is a problem with the rear (post catalytic converter) oxygen sensor.
What P0140 really means
When the oxygen sensor stops producing voltage, or goes beyond the calibrated voltage, the computer declares a problem. In the majority of vehicles, the rear oxygen sensor acts only as a monitor for catalytic efficiency and does not have any input in fuel management. When the catalytic converter works properly, the oxygen in the exhaust stream measured by this sensor should produce a steady voltage on the rich side of the scale, which indicates the converter is using up oxygen in chemical activity.
What causes a P0140 code?
The rear oxygen sensor acts as the pollution police. Other than telling that the catalytic converter is not doing its job, in most cars, there is little else for it to do. You will not notice any difference in the way the car performs. These sensors go bad over time, especially if the car has small oil or coolant leaks that contaminate the exhaust, so there is little you can do to prevent failure.
What is the severity of a P0140 code?
Minor, however a proactive approach might catch small issues in the converter before they become big.
What repair(s) are needed to resolve a P0140 code?
Replace the rear oxygen sensor. Although the rear oxygen sensor does not change performance in most cars, a mechanic should diagnose the problem. A bad reading in the rear oxygen sensor might be caused by a failing catalytic converter, or it might conceal a converter beginning to fail.
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Service article written by an ASE Master Technician