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Genchi, I have a 2003 Toyota Corolla (159000 ***** *****)

Customer Question
Hi Genchi, I have...

Hi Genchi, I have a 2003 Toyota Corolla (159000 ***** *****) with check engine light on (codes p0171 & p0430) in the past. Since then I have replaced the MAF with new Denso MAF, used cataclean to clean the CAT, used seafoam through the throttle body and erased the codes couple of times. As such there was/is no misfiring, rough idle or driving problems. This car was driven quite a lot in freezing temperatures of NY but 6 months back I moved to California and have driven the car around 700 ***** ***** but the problem still exists. Currently, there is no CEL but code p0171 shows as faulty and p0430 comes and goes. Since I have replaced the MAF, I was wondering what else could be the reason for these codes? Thanks

Mechanic's Assistant: Thanks. Can you give me any more details about your issue?

I need to pass the smog test and that is why I want to fix these codes

Submitted: 2 years ago.Category: Toyota
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Answered in 42 minutes by:
2/25/2016
Toyota Mechanic: Toyota Service, Toyota Expert replied 2 years ago
Toyota Service
Toyota Service, Toyota Expert
Category: Toyota
Satisfied Customers: 5,211
Experience: 30+ Yrs of auto experience, Award Winning Svc. Manager; Just Answer Customer's Choice Award Winner
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Hello. Welcome to Just Answer. Please allow me to assist you. You have did everything that a good shop would do thus far. I commend you on your skills. The next thing that I would be doing, if this car was in my shop, would be replacing the intake manifold gasket (part number 17171–22060). There is a known history of the intake gaskets allowing air to leak past the gasket into the cylinders. A P0171 code is a lean condition. Assuming you have no other leaks on the intake, the gasket is most likely your cause.

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Toyota Mechanic: Toyota Service, Toyota Expert replied 2 years ago

Please confirm that you have received my input. If you have any questions, please feel free to ask. Thanks.

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Customer reply replied 2 years ago
I believe that seems to be the next best option to replace the intake gasket manifold. But can this also be the reason for getting P0430 code. Is nt it that leaking intake gasket manifold only raises P0171. I am asking because some mechanics suggested to replace the catalytic converter just by looking at the codes that were generated.
Customer reply replied 2 years ago
Further, how can I check the leaks on intake as I have not checked that yet.
Toyota Mechanic: Toyota Service, Toyota Expert replied 2 years ago

You can get P0420 and P0430 codes (cat codes) and there is nothing wrong with the cat. The sensors read what is going into and coming out of the cat. If you have faulty data going in (too lean), your output logic on the sensor is going to be compromised. I would never recommend a cat based solely on a p0430 code. Cats rarely fail these days. Run it too lean and you could damage it, yes. This sounds more like a bad oxygen sensor than a bad cat, but that is moot right now because the engine is too lean to start with.

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Toyota Mechanic: Toyota Service, Toyota Expert replied 2 years ago

I have checked for intake leaks on these cars; tried every trick in the book, while having a new gasket ready to install. I have yet to find where the leak is (most likley, it is on the bottom of the manifold). The replacement gasket is different than the original. Toyota knows of the gasket failure issue and has since redesigned the gaskets.

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