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Lean bank 1 sensor 1...could it be catalytic converter?

My car is at the shop right now and I just need some advice so I don’t get taken advantage of.

My car is a 2002 toyota camry 4 cyl

I am getting a code P0171 for lean bank 1 sensor 1. I have replaced the fuel filter, oxygen sensor, PCV valve, and cleaned the MAF sensor. Still getting the code, and need to get smogged soon, so I took it into the shop.

He is going to test the fuel pressure tomorrow and says it MAY be a clogged catalytic converter. However, I read that a bad catalytic shows symptoms such as bad fuel economy (my car still gets amazing fuel economy) and loss of power (I’ve noticed no loss of power).

I use a cheap sensor that shows me codes and it’s never given me a catalytic converter code, and I’ve had P0171 for over a year now. However, is it possible that my cheap sensor doesn’t have that particular code programmed into it?

Just looking for advice in case he does come back and say it’s the converter. Suggestions on what to expect would be helpful as well. Thanks.

I would be looking at the fuel injectors. Unlikely to be the cat.

Had that code on a different make, ended up being bad orings on the plenum. Cat not near the top of my list either.

Here’s a link to a site with some brief comments on this code.

I would guess that you either have a clogged fuel injector, or you have a vacuum leak.

Check all the vacuum hoses for cracks or breaks. They are already 14 years old and many dry out and crack.

I wouldn’t consider the cat to be bad from your info.


There are so many things that can cause a P0171 fault code that to hazard a guess here would be more confusing than helpful.

I can tell you that a good technician with a proper scan tool (think thousands of dollars) could drive your car for a few miles, record some data, and be 90% sure of where the trouble is before even leaving the driver’s seat.

A mechanic that will tell you “it may be a catalytic converter” without even running some tests is like a doctor saying “you may need surgery” before even getting you into the exam room.

I wonder if the mechanic has even bothered to hook up an evap/smoke machine

I wonder if he even has one, or if he does, if he uses it often

That will take just a few minutes, and will quickly show if you have any intake/exhaust/vacuum leaks

I specifically mentioned exhaust leaks . . . because an exhaust leak ahead of the upstream oxygen sensor can result in P0171. And many guys don’t consider that

Personally, I’m leaning towards some kind of an intake or vacuum leak

Personally, I doubt any of the injectors are bad, based on OP’s assertion that the car has normal power and fuel economy

The first thing that pops into my overburdened brain when I hear a Lean Condition is a Vacuum leak. As @Yosemite and others already pointed out. I would also have to agree with @asemaster on the diagnosis being postulated by your “Mechanic”… Lots of ways to remedy this situation…you just need to find the right people…


Had a similar issue with an 03 Corolla. I replaced the O-Ring that goes around the MAF sensor and it cleared it up.

He mentioned the fact that my car starts slow, and that lead him to believe that there could be a loss of fuel pressure. Somehow the catalytic converter got mentioned though I’m not sure how that would cause a lean code.

P0171 for lean bank 1 sensor 1

“bank 1” is meaningless for this engine, it’s used to differentiate which side of a V6 or V8, but you have a straight 4.

“sensor 1” usually means the before-the-cat O2 sensor. There’s an O2 sensor both upstream of the cat, and downstream. This code is for the upstream one.

“lean” means there’s too much air (or O2) getting into the engine coming from somewhere or another, and it is having a difficult time dealing with all that extra air.

That particular O2 sensor’s job is to assist the engine computer in setting the correct air to fuel mixture to make the engine run the smoothest and least emissions possible. It uses that sensor’s reading in conjunction with information from other sensors, especially the MAF sensor, the coolant temp sensor, and the ambient temp sensor. What it is saying is that it is having to inject more gas than it thinks it should, given the readings from all the sensors other than the O2 sensor.

There’s three common problems to look for first

  • unmetered air is getting into the engine’s intake manifold. All the air that gets into the engine is supposed to go through the MAF sensor, where it is measured as it passes by. If extra air leaks in somehow or another, this confuses the engine computer. Vacuum leaks, torn intake boots, compromised gaskets, or even just the intake manifold bolts are loose are all possibilities. Shops have simple ways to test for all of these.

  • air is getting into the exhaust stream upstream of that O2 sensor. You’d think if you had an exhaust leak, it would only go one way, from the exhaust system out to the air. But the way the pistons move both directions, outside air can also be drawn into the exhaust system if there’s a leak. The engine computer assumes everything that crosses the path of the O2 sensor came from the combustion chambers, so it gets confuses if any outside air get into that stream. Cracked exhaust manifolds on Toyota 4-bangers are not an unheard of problem here, and could cause this symptom. Or a bad exhaust manifold gasket, among others.

  • The O2 sensor isn’t working correctly. Those are pretty robust, and from comments here, failures are very uncommon, esp O2 sensors less than 10 years/150K miles old. I’d suspect a broken or intermittent wire between the sensor and the engine computer before I’d suspect the sensor.

If you remain concerned about the cat, ask the shop to do a cat back-pressure test. If the cat was clogged I suppose this symptom could result. But if it was just not doing a good job of cleaning the exhaust of pollutants, that would usually trigger a code, but not this one.

Intake manifold gasket.

If I remember correctly, these also have a plastic intake. These can and do crack, not real often but it can happen.

Would the intake leak show up during a smoke test?