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RICK, ASE Certified Technician
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What is the fix for a P0420 check engine light?

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what is the fix for a P0420 check engine light?
HiCustomer Thank you for asking your question on JustAnswer. The other Experts and I are working on your answer. By the way, it would help us to know:
-What year is your Car?
-What make and model is your Car?
-How many miles are on your Car?
-What type of engine is in your Car?
-What have you already tried?

Thank you again for trusting us with your problem. Please reply as soon as possible so that we can finish answering your question.
Customer: replied 8 years ago.
My 2002 Yukon XL has 138,000 Miles on it. The engine is a Vortex 5.3 Litre (1500). I have not tried to do nothing with this problem because I am not that smart of a mechanic. But one mechanic that worked on my truck when the fuel pump went out said to check the TPS. That is all I have gotten in the form of an idea from a mechanic. Is this something that could give me this code number?


sorry code P0420 is a cataletic convertor efficiency code, only way to get rid of it is replace that expensive convertor


DTC P0420 or P0430

Circuit Description

In order to maintain low emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the engine controls system uses a 3-way catalytic converter (TWC). The catalyst within the TWC promotes a chemical reaction, which oxidizes the HC and CO that are present in the exhaust gas. This reaction converts these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx which converts into nitrogen. The powertrain control module (PCM) monitors this process using heated oxygen sensor (HO2S) bank 1 sensor 2, which is located in the exhaust stream past the TWC. HO2S bank 1 sensor 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to effectively convert the exhaust emissions. If the catalyst is functioning correctly, the Bank 1 of the HO2S 2 signal will be far less active than the signal that is produced by Bank 1 of the HO2S 1. This indicates that the TWC oxygen storage capacity is below an acceptable threshold.

The PCM performs this diagnostic test at idle. When the conditions for running this DTC are met, the following occurs:

  1. The PCM captures the current rear HO2S rich-to-lean status.
  2. The air-to-fuel ratio transitions from rich to lean or from lean to rich, depending on the capture rear HO2S rich-to-lean status.
  3. The air-to-fuel ratio transitions a second time opposite the first air-to-fuel ratio transition.
  4. The PCM captures the response time of the front and rear HO2S when the air-to-fuel ratio transitions occur. The HO2S response time goes from less 300 mV to more than 600 mV, and from more than 600 mV to less than 300 mV.
  5. The PCM measures the time necessary for the rear HO2S voltage to cross a reference rich-to-lean threshold, minus the time necessary for the front HO2S voltage to cross the same rich-to-lean threshold. The difference between the front HO2S time and the rear HO2S time is the oxygen storage capacity of the catalyst. This DTC sets if the time exceeds a predetermined threshold
  6. Before you replace the TWC, correct any conditions which may have damaged the catalyst.Replace the TWC, see replace convertor next page.
Customer: replied 8 years ago.
So you are telling me that the only way to fix this and clear this code would be to replace the Catalyst converter and there may stlll be something else wrong with the motor that caused the converter to fail?


thats correct, normally fuel system giving vehicle too much fuel or not enough, damaged convertor, maybe because fuel pump went out, running lean for too long.

RICK and 7 other Car Specialists are ready to help you
Customer: replied 8 years ago.
Thank You very much for your time and help.

hi ,

your welcome