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2017 SS Sedan 6.2L
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This might be a helpful place to start your troubleshooting.

Common causes of the P0420 code?
  • Damaged muffler or leaks in the muffler.
  • Damaged exhaust manifold or leaks in the exhaust manifold.
  • Damaged exhaust pipe or exhaust pipe leaks.
  • A misfire in the engine.
  • Oil contamination in catalytic converter.
  • Faulty catalytic converter (most common).
  • Faulty engine coolant temperature sensor.
  • Faulty front oxygen sensor.
  • Faulty rear oxygen sensor.
  • Damaged oxygen sensor wiring.
  • Oxygen sensor wiring that is not properly connected.
  • Damaged oxygen sensor connectors
  • A leaking fuel injector.
  • High fuel pressure.
 

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Same problem with me they told me to replace the catalytic converter but I don’t think that’s the problem. I wanna say it can be a high fuel pressure. Because when the car is warm up it still takes a couple Turns then normal to turn on. That means the engine is flooded with fuel. My guess everyone says same thing. I been on this BS problem form a year. Tried everything besides the fuel pressure / fuel pump/ fuel regulator. Try that and tell me how it goes. I ain’t no mechanic but also if the fuel pressure is high means rich fuel. Which means too much for catalytic converter to burn so it stays there maybe effects the 02 sensors ?
 

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Hello all… hoping someone will shed some light on this thread. What was the final verdict?
I have a 2015 Malibu 2.5L.
The dreaded p0420 keeps showing up after roughly 20 miles.
New cat converter, new acdelco o2 sensors, new injectors, new high fuel pressure pump, new intake manifold and and all gaskets. Also replaced the coolant temperature sensor. And finally at last, the pcm was reflashed. Drove it home again yesterday and CEL came back on after roughly 20 miles again.
Any help will be appreciated
 

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Does your car have California emissions? Will say so on the under-hood sticker. Many states have CA emissions, so it doesn't have to be a CA bought car. Most of the cars sold in northeast coast states require CA emissions for example.
Many aftermarket cats will not be clean enough to satisfy the computer. Factory cat is the only one that will do the job.
Been there when my brother took his Impala to a local shop, and they put a non-factory cat in his CA emissions car in NJ which is a CA emissions state. Lots of issues with the cheaper aftermarket cats in CA emissions cars.
 

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my vehicle is California emissions. The cat I had install is aftermarket. I have read that factory cat might be the only way. I might be out of luck as I no longer have the oem cat on the car for the core exchange.
 

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Just use the aftermarket cat on exchange. Cat for a cat. They don't ''rebuild'' them, they just gut the core for scrap and the palladium and other metals.
 
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