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Lean bank 1 sensor 1...running out of ideas

I have a P0171 code (lean bank 1 sensor 1) on my 2002 toyota camry 4 cylinder. I don’t have any other codes except for an unrelated P1780 neutral safety switch code. It has had both codes for over a year.

I have replaced the fuel filter, the upstream o2 sensor, and the PCV valve. I also cleaned the MAF sensor. The mechanic performed a smoke test and found no vacuum leaks. Did a fuel pressure test and it’s normal. Mechanic thought it would be fuel pressure because the car starts a bit slow when cold, but nope. The car runs great and still gets great gas mileage. I’ve never experienced a misfire in my 12 years of owning the car. I am asking because I am at a loss as to what it could be. Thanks.

Also, I had a mechanic test the MAF sensor and he said it checked out.

Has the intake hose from the MAF sensor to the throttle body been inspected for cracks/leaks?


Two threads with the same subject now.

I still think it’s the intake manifold gasket.
Smoke test doesn’t apply enough pressure differential to deform the gasket and make it leak.
It’s a common problem with this vintage 4-cyl Toyota
There is an updated gasket made of orange silicon instead of black nitrile rubber.
The gasket is less than $10, so it’s mainly labor, maybe 45-60 minutes.

I think you posted this same problem a few days ago, right? Have you tested all those ideas presented there, and still can’t get that pesky code to go away?

hmmm … here’s what I’d do next if it happened to my Corolla

  • trace the wire from the O2 sensor to the computer’s connector, make sure that’s a low ohm solid connection the entire way. also make sure the O2 sensor’s ground system is properly connected.

  • fuel injector balance test. If some of the injectors aren’t putting out enough gas b/c they are clogged, that could look like a lean problem. i.e. too much air is the same as not enough gas from a fuel/air mixture point of view. And if it is just one or two of the injectors that are clogged, there’s no way for the computer to compensate. You could try some fuel injector cleaner too. I had a drivability problem w/my Corolla one time, where it was jerking during low speed turns, like turning in neighborhoods, and I solved that with a fuel injector cleaning treatment.

Circuitsmith- Thank you for your recommendation.

Problem was not the manifold gasket. There was a slight blockage of the air intake, somehow it caused a lean code. Removed the blockage and no more lean code.

Can you elaborate on what and where the blockage was?

And all the time, he wondered where that right glove went!!!


Thanks for posting what you discovered OP. You’d think a blockage at the air intake would cause too little air (a rich code) rather than too much air (a lean code), but the blockage probably disturbed the flow patterns in the air intake path and affected the ability of the MAF to get an accurate reading.

As an analogy, it’s like trying to measure the entire volume flow of a river by measuring the water velocity at one spot on the surface using a paddle wheel. The water velocity in a river varies by distance from the shore and how deep the water is. But on most rivers you could estimate the entire flow by measuring at a single point and calculating by trial-and-error a single point velocity to entire river flow proportionality factor. But if some blockage upstream later causes less water than usual to flow right where the paddle wheel is, it will make it seem like there’s less water flowing in the river than there actually is.