To control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx to nitrogen.

The converter also has the ability to store excess oxygen and release the stored oxygen to promote these reactions. This oxygen storage capacity (OSC) is a measurement of the catalysts ability to control emissions. Code P0420 will set when the system is not efficient. The powertrain control module (PCM) monitors this process using a heated oxygen sensor (HO2S) located in the exhaust stream past the three-way converter. When the catalyst is functioning properly, the HO2S 2 is slow to respond to a large change in the HO2S 1 signal. When the HO2S 2 responds quickly to a large change in the HO2S 1 signal, and efficiency of the catalyst is considered to be bad and the malfunction indicator lamp (MIL) will be illuminated if subsequent tests also indicate a failure.

Almost all of the time, a failed converter is the problem when this code sets. Many times other codes will be set, especially if the engine lacks power because the catalytic converter is plugged. Misfire and EGR valve codes may also accompany P0420. The EGR and misfire codes would be the result of a plugged converter, not the cause of the code P0420. Although, running with a misfire for a long period of time will damage the converter, thus causing the P0420 DTC.

Fixing an engine misfire is the first thing that needs to be done. Once that is repaired, clear the codes and test drive the car. The catalytic converter code may not return since the condition causing that code has been fixed. A flashing check engine light is a government mandated signal that the misfire is bad enough to cause converter failure. This should be the first problem that gets repaired before more expensive repairs are needed.

Most exhaust repair shops will have a great price on replacement, but be careful. They usually use low quality parts and this can actually result in more expensive repairs because a cheap converter can still give the code P0420 since it is not cleaning the exhaust as good as the computer wants to see. So you may be fooled that that was not the problem and get mad at your mechanic. We would not want that ! Quality parts should always be used. They are more expensive but will save money i the long run and also keep a clean environment for us and our kids.


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  • Guest - Keith

    I have replaced the cats and the light keep s coming back on