Its most likely a rear sensor. Is the car running normal, good acceleration.....?
A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in the exhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.
The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (unburned hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.
The OBD II system can't actually measure the concentration of HC or CO in the exhaust, so it compares the upstream and downstream O2 sensor readings to estimate how well the catalyst is actually doing its job of removing pollutants from the exhaust.
The upstream O2 sensor will typically show a lot of switching activity because the engine computer is constantly adjusting the fuel mixture between rich (more fuel) and lean (less fuel). When the engine is first started, the catalyst is cold and doesn't do much. During this time, the switching activity of the upstream and downstream O2 sensors are essentially the same because nothing is happening inside the converter.
When the converter reaches about 600 degrees F, it is hot enough to start reacting with the gases in the exhaust. This is called the catalyst's "light off" temperature. The converter now starts to clean the exhaust and remove the pollutants. This causes a sudden drop in the switching activity of the downstream O2 sensor, and the sensor's output voltage levels off to an average reading of around 0.45 volts. This tells the OBD II system that the catalyst is doing its job and that everything is fine.
The OBD II catalyst monitor only runs under certain operating conditions, which typically require a combination of city and highway driving on the same drive cycle. If the catalyst monitor has run and no faults are found, the converter should be functioning properly and the vehicle should be in emissions compliance. But if the catalyst monitor runs and finds too much switching activity in the downstream O2 sensor after the converter is hot and the vehicle is being driven, it may set a P0420 code and turn on the Check Engine light.
2012 CTS-V Coupe, 4k miles
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2005 Bonneville GXP, 200k and SOLD!