I wanted to share my story and ultimate fix for this issue. Sorry this is going to be long winded but it has been going on for 3 years. This is a 2000 Toyota Avalon V6 with 128,000 miles on it.
3 years ago my CEL came on and it had a code of P0171 and P0135. Bank 1 too lean and Oxygen O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
At the time it was my dads car so he took it to a local indie shop. The where unable to replace the O2 sensor because it “was spinning in the manifold” they quoted us 800+ in labor just to remove the manifold to fix it. Dad declined said he had no driveablity issues. Well a year later I own the car and it is stumbling all over the place. So I take it to another shop knowing that I am walking into a huge bill for this one sensor. To my surpise they were able to replace it for less than $400. Issue resolved? I think not. About 30 days later P0171 came back on with no Drive issues. I just dealt with it till I had the money to find out what was going on. During this time I also had a leak/seep in my valve cover gaskets. Awesome, according to alldata the number one cause is air induction issues so I can change the gaskets and the air intake manifold gaskets as well as throttle body gaskets all at once and check for vacuum leaks. So I set off and wham broke a bolt in the head from the valve cover. I know sucks right??!! Now the oil leak at the rear bank is much worse (pouring out) so off to the shop it goes for that freaking bolt. So after a head gasket job and a machine shop charge that I thought was crazy I now no longer have a leaking gasket and no CEL. Yay they fixed it right??? Wrong a week later CEL came on. Same Friggin code. I am pissed. Shop says it is not related to the job they did so they want to rediagnose and charge more money. So I let it be.
A year goes by and now I notice issues with MPG. So I start searching online and come across Eric the Car Guys youtube videos. I must admit that I am hooked on them. I must of watched every one of them although almost none of them have to do with my current issues. I did find one about vacuum leaks that I try on my car to no avail. So I dont think I have a vacuum leaks.
I reach out to another forum before I come here cause even though it is in the videos I didn’t really realize it was here. A Toyota Tech said 99% of those problems have to deal with mass air flow sensor issues. No DTC pointing to that.
Any ways long story short. Took the MAF out to check it and clean it. Guess what CEL is gone has been gone for quite a while and MPG is back to normal.
All that pain and money for what ended up being a dirty MAF. Granted the O2 sensor needed to be replaced and the valve cover gasket was seeping oil and needed to be change but I would have waited on the gasket till it got worse and then I would have been smarter about torques (Watched enough videos to know better I now own a Torque wrench) and maybe not broke that friggin bolt. You live and learn. banana:
My only question is why did the MAF only cause one bank to run lean instead of both?
Sorry this is so long winded but I thought I would share my struggle.
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