I got this code recently “bank 1 system to lean”

I cleaned the maf, code went away and the. Returned. I replaced an AF SENSOR years ago when I got this code and I’m thinking it’s either that sensor again.

Can someone tell me which sensor this would be, I just want to make sure it’s the right one?

My dad recently got the same code is in his 01. It ended up being that his gas cap wasn’t on tight enough.

Check all the vacuum hoses, too.

Is this a cali emissions truck, or federal emissions truck?

No offence, but this doesn't make any sense. P0171 means that the engine is either getting too much air or not enough fuel. A loose gas cap would throw an Evap code, not a Lean code. If you cleared the code then it might not necessarily come back right away. How long ago did this happen?


As far as the OP's question, P0171 in my experience is not usually a sensor issue but it's possible.
The most common issue is a cracked/broken intake or vacuum hose AFTER the MAF sensor - this sensor reads the incoming air and tells the ECM how much fuel to spray from the injectors, so if there is a leak after the MAF then there is unmetered air getting into the combustion chamber - too much air/not enough fuel.

Now, since there is only the single code for bank 1, there are a couple different things. Either both banks are lean, but the data list parameters haven't quite hit the limit to throw the code for bank 2 - OR the lean condition really is limited to bank 1 after which point I would check the intake manifold gasket for bank 1 to make sure it hasn't torn.

These aren't the only options but just food for thought, obviously check for any exhaust leaks close to the O2 sensors.

Good luck!

Forgot to mention....TERRIBLE GAS MILEAGE

I have been chasing this code since October and have done almost everything to try and get rid of it. I have changed out the upstream o2 sensor and replaced the MAF with a denso one. I have checked all vacuum lines and they are good to go. I also have cleaned everything thoroughly to ensure no grime in places it shouldn't be. Only thing left for me to do is change out the downstream o2 sensor. Mine is supercharged

I was getting this one for awhile as well. The first time I cleaned the MAF sensor, it came back. I had forgotten to clean the other two small wires inside the sensor. Once I did that, it has stayed off for quite awhile.

He has an A/F sensor, so Cali.

Doing good until the bold. Our 3G 4Runners only have Bank 1 exhausts, so only the single lean code. There is no Bank 2 lean code. 4th gens have dual exhaust and a Bank 2 lean code.

P0171s are indeed usually MAF issues or leaks. But after ruling those out, I did clear mine with a new A/F sensor one time. Sometimes cleaning is not enough and you need a new MAF. I hate P0171s.

Here are possible issues.

Diagnostic Codes
P0171

Confirmed Fix
225 - Mass Air Flow (MAF) Sensor
9 - Engine Air Filter, Mass Air Flow (MAF) Sensor
4 - Oxygen (O2) Sensor(s)
2 - Fuel Pump
1 - Front Oxygen (O2) Sensor
1 - Vacuum Leak
1 - Fuel Injector(s), Mass Air Flow (MAF) Sensor
1 - Cruise Control Vacuum Line
1 - Intake Manifold, Mass Air Flow (MAF) Sensor
1 - Mass Air Flow (MAF) Sensor, Throttle Body Cleaning Procedure
1 - Mass Air Flow (MAF) Sensor, Mass Air Flow (MAF) Sensor O-Ring
1 - Exhaust Pipe, Mass Air Flow (MAF) Sensor
1 - Air Intake Tube
1 - Mass Air Flow (MAF) Sensor, Throttle Body
1 - Air Fuel Ratio (AFR) Sensor
1 - Engine Vacuum
1 - Oxygen (O2) Sensor, Bank 1 Sensor 2

i had this code on my 2007silverado classic and it ended up being a vacuum leak in the break booster. Replaced it and have not thrown a code since.

Id run a smoke test if you could. That's how we found the vacuum leak in my brake booster that plus a nice hiss when they engine was turned off coming from the booster.

I wish I could get something like this for a P0172 code. where did you find this information?

Fair enough! In my defense, I don't actually own a 3rd gen yet LOL, so I was not aware of the single exhaust. I was just going off my experience as a Lexus master tech for the last 5 years. So there is only one cat and one sensor? Unlike the 3MZ engine which as 2 cats, one off each exhaust bank.

It very well could be a bad A/F sensor, just wouldn't be the first thing I would replace. Trust me, I hate P0171s as well! It's either really simple or very complicated.

Good point! I had a car I was working on in the shop once but it had both banks lean codes. All the data list numbers looked way off, all hoses were perfect, but the red flag was the brake pedal was very stiff due to a bad brake booster leaking air back into the engine.