Getting Rid Of Code P0420 With A Little Spring Cleaning
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Getting Rid Of Code P0420 With A Little Cleaning (VIDEO)

A good cleaning might be all you need to get rid of code P0420. This video is sponsored by Rislone.




We just pulled a P0420 from a vehicle indicating the catalyst system efficiency for one of the banks is below the threshold. What’s next? Could it be the oxygen sensor, or could it be the Cadillac converter? Understanding why the P0420 code was set and what further test must be performed to determine the root cause of the problem are just as important as replacing parts.

A P0420 code is set when the upstream sensor and the downstream oxygen sensor sense the catalytic converter is not doing its job. Basically, the amount of oxygen going into the converter is not being burned, and it detects it through the downstream sensor.


Oxygen is the key here. Modern Engine Management Systems leave a little bit of extra oxygen for the catalytic converter to burn the unburned hydrocarbons and break down harmful pollutants. If the oxygen is not being used as expected or the level of oxygen coming out is the same, the Engine Management System knows the converter is not working as designed.

These oxygen levels are monitored by the oxygen sensors mounted before and after the catalytic converter. These are otherwise known as the upstream and downstream oxygen sensor.

For a catalyst efficiency code like P0420 to be set, a number of criteria must be met. For example, the oxygen or air/fuel ratio sensors must see a reduction in efficiency of the converter. In other words, the oxygen levels before and after the converter, they don’t change or they’re not withinside the specified levels, well, it’s going to set a code.


But it is not an automatic pass or fail. The oxygen sensors need to see this loss in efficiency over a number of drive cycles and under certain conditions. This is why it might take a few hours or even a few days for the light to come back on after you clear that efficiency code, and this is why it may take time for that code to come back.

The converter has an efficiency rating that is computed by the Engine Management System. This number rates the amount of reduction that is occurring in the converter and its ability to even store oxygen. But the efficiency of the converter is tied to the fuel trim of the engine.


Most engines will minutely alter the short-term fuel trim to replenish oxygen in the converter to add fuel or reduce the amount of fuel. This helps to keep the converter in the correct temperature range to achieve the most efficient operation.

In some cases, the engine and catalyst converter might need a little cleaning to get rid of a code P0420. It could be a case where the injectors are clogged and the spray pattern might be less than optimal. In other cases, the catalyst brick might be a little contaminated. For vehicles with a broader line of efficiency numbers, it might be advantageous to have a good cleaning with an intake treatment and a cleaner that might help to extinguish that check engine light for a code P0420.


I’m Andrew Markel. Thank you very much.

This video is sponsored by Rislone.

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