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Discussion Starter · #1 ·
Hello there, I owned a 2010 Buick Enclave with 138K miles. I am the sole owner of this SUV since it was first purchased. Recently it had a P0420 code, bad mpg, and rich gas smell at the exhaust. There is no check engine lights or any performance issue besides bad mpg. I replaced the Cat bank 1 and was able to resolve the p0420 code. However, bad mpg and the gas smell at the exhaust remain. It averages 10-14mpg as supposed to the usual 17-21. I already replaced;

Throttle Body
High-pressure fuel pump
6 coils and 6 spark plugs
Mass Airflow Sensor

I already cleaned the valves - and replaced the air filters as well. I'm running out of options here. Just wondering what else I can check - bad fuel injectors? exhaust or vacuum leak somewhere? Please help.
 

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It may be possible that you have carbon build-up on the valves, which is not allowing them to operate properly, causing misfires. The misfires may not be consistent enough to trigger the misfire check engine light, but enough to damage the Cat. Carbon build-up is, unfortunately, common in our Direct Injection engines.
 

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What brand catalytic converter did you purchase? Cheaper aftermarket cats cost less for a reason unfortunately. Less precious metals (up to 50% less in some cases) to convert the exhaust gases.

I maybe wrong but I think the OEM cats hover around 96-98% efficiency and the computer trips the p0420 at 90% or 92%. I remember reading it somewhere in an article about California drivers only being able to buy OEM. Cheap aftermarkets barely clear 92% and they all fail California emission standards.
 

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Discussion Starter · #7 ·
i live in California - only CARB compliant Cats are avail here - meaning the one I have is OE ( bought from the stealership) cost me $1400

What brand catalytic converter did you purchase? Cheaper aftermarket cats cost less for a reason unfortunately. Less precious metals (up to 50% less in some cases) to convert the exhaust gases.

I maybe wrong but I think the OEM cats hover around 96-98% efficiency and the computer trips the p0420 at 90% or 92%. I remember reading it somewhere in an article about California drivers only being able to buy OEM. Cheap aftermarkets barely clear 92% and they all fail California emission standards.
 

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Spark plugs and ignition coils are good?

Check the vapor cannistor purge valve. That's a big mpg loss tho, probably isn't the problem.

A scanner showing live data would help you narrow it down. The LTFT/STFT.
 
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