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Ive had this problem over a year and i cant fix it. I had change my intake gaskets, coils,spark plugs , air filter , clean the MAF sensor, both 2 of bosch o2 sensors , changed my CAT converter because it had a p0420 so it got cleared and still the check engine light doesnt turn of for the p0171 which is running lean! The only thing I haven't changed is the Fuel injectors and the fuel filter. My Long Fuel trim is showing 40.1% when it should be below 10%. Please help!! Any suggestions would be great.
 

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Seems that 2001 Corolla has common reports of MAF sensor causing this problem and it's very often the case that a P0420 is involved. Bosch is generally a quality product, but for some reason Bosch products don't match well with Toyota systems. I'd use Denso brand O2 sensors instead. Try clearing the code with a scanner and disconnecting the MAF and see if the code does or does not come back. If it does not come back, then probably your MAF is toast and cleaning it again won't help. Check the PCV valve and its hose. That is another cause. Yes, it could be faulty or leaking fuel injectors.

A smoke test for intake air leaks would help to find a hose that is compromised. Check all your intake hoses for signs of being compromised, which includes just being brittle/hard in addition to tears/craks/disconnections.

Unless you're experiencing severe acceleration problems, like bucking, I've never seen a need to replace fuel filters on Toyotas. I used to replace them automatically whenever I bought a used car, but now I skip that. It's almost always been unnecessary. If the car sat for a really long time at any point, then yeah I would replace the fuel filter.

Check for exhaust leaks as well, as this can cause this problem. Check that the cat replacement was done correctly and there aren't any leaks. To find exhaust leaks, I use cheap bubbles solution from the dollar store (comes in a big bottle) and put it in a sprayer. Spray everything from the exhaust manifold going rearwards. Make the idle go to 2K RPMs and look for big bubbles. If you see bubbles, then you've found a leak.

Your lean condition could have led to the car's computer dumping more fuel into the system, which is what could have killed your cat. Check for valve cover gasket leaks as well. I know that isn't normally thought to contribute to this, but I think it's a remote possibility. Here are the typical causes, though:

295686
 

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I had this EXACT same issue on my car. It's the fuel injectors. You can either clean them yourself, or buy new ones from rock auto. They're very easy to swap out too.
 

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I agree with the statement that the injectors are highly suspect. Consider the number of cycles and the environment. Say 3 k rpm 4 cyl, 6000 cycles PER MILE. 200k miles, how many times have those injectors fired. 6000X200000=12 with 9 zeros after that, 12 billion cycles. Pull them out and put them in a machine that fires them so you can see the pattern and volume of fuel delivered.
Or try a bottle of techron FIRST. i can personally attest to what that can do in 50 miles or less, and it requires no more than a bottle of techron.
It will not fix everything, but it might fix your problem and it cost you a bottle, versus a litany of wasted money and effort.
That's a no brainer in my book.
When you gave a customer that advice and it worked, you had a customer for life.

Watch it at 12:33 and look at the difference if volume that can not be adjusted by the CPU. You have a lean cylinder or more than 1 with no solution but cleaning or replacement
 

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Discussion Starter #5
I had this EXACT same issue on my car. It's the fuel injectors. You can either clean them yourself, or buy new ones from rock auto. They're very easy to swap out too.



Thanks! I have resolved my issue. It was the 4 fuel injectors all this time and I had never change them since 2008.
 

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Discussion Starter #6
Seems that 2001 Corolla has common reports of MAF sensor causing this problem and it's very often the case that a P0420 is involved. Bosch is generally a quality product, but for some reason Bosch products don't match well with Toyota systems. I'd use Denso brand O2 sensors instead. Try clearing the code with a scanner and disconnecting the MAF and see if the code does or does not come back. If it does not come back, then probably your MAF is toast and cleaning it again won't help. Check the PCV valve and its hose. That is another cause. Yes, it could be faulty or leaking fuel injectors.

A smoke test for intake air leaks would help to find a hose that is compromised. Check all your intake hoses for signs of being compromised, which includes just being brittle/hard in addition to tears/craks/disconnections.

Unless you're experiencing severe acceleration problems, like bucking, I've never seen a need to replace fuel filters on Toyotas. I used to replace them automatically whenever I bought a used car, but now I skip that. It's almost always been unnecessary. If the car sat for a really long time at any point, then yeah I would replace the fuel filter.

Check for exhaust leaks as well, as this can cause this problem. Check that the cat replacement was done correctly and there aren't any leaks. To find exhaust leaks, I use cheap bubbles solution from the dollar store (comes in a big bottle) and put it in a sprayer. Spray everything from the exhaust manifold going rearwards. Make the idle go to 2K RPMs and look for big bubbles. If you see bubbles, then you've found a leak.

Your lean condition could have led to the car's computer dumping more fuel into the system, which is what could have killed your cat. Check for valve cover gasket leaks as well. I know that isn't normally thought to contribute to this, but I think it's a remote possibility. Here are the typical causes, though:

View attachment 295686

Thank you. I have resolved my issue. It was the 4 fuel injectors. My car runs like new again!
 

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Great. How did you diagnose fuel injectors? Also, did you just replace them, or only the O-rings/gaskets, or did you have them serviced/cleaned?
 
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