[Solved] Mass Air Flow Sensor - Bank 1 or Bank 2 Running Lean

All of a sudden my truck shows the infamous P0171 code - Bank 1 running lean. I have a couple of questions:

1. What determines if the problem is a Bank 1 running lean or a Bank 2 running lean?

2. Has anyone replaced the MAF sensor with a Genuine Toyota Part because of a P0171 code and have it turn out NOT to be the problem?

3. If so, what was the problem?

I'll be cleaning the sensor, but in the meantime if anybody can enlighten me I would appreciate it.

 

1. Bank 1 is the 02 sensor located up-steam of the cat. Bank 2 is after.
2. I think some have but cleaning it is your cheapest alternative. Search "Volvo MAF tacomaworld" in google and behold some really good knowledge on a retrofit
3. I had this happen and it was a dirty MAF. I cleaned it and consequently had a bad coil pack. Fixed my problem. Could be yours and could not be. It's a crap-shoot sometimes (or throwing parts at it) lol

 

My experience with this annoying ass code was that it was a vacuum leak, and the mechanic that figured it out told me that's almost always what a P0171 points to.

Cleaning the MAF is a good start since that's easy to do and a good idea in any case, but I would check carefully for vacuum line leaks anywhere after the MAF sensor.

In my case, they couldn't find a leak right away so my MAF sensor was replaced along with my o2 sensors, they even replaced the spark plugs/wires (truck was under warranty at the time so I let Toyota throw parts at it). Nothing got rid of the code.

Finally a master tech was called in and he discovered a small vacuum leak on the back of my supercharger, plugged that and the code disappeared. He was the one that told me that this code is almost always vacuum related.

 

Well if it is a 4 cyl then it has 1 bank. if it is a V6 it has 2 banks 3 cyl on bank 1 and 3 cyl on bank 2.

So if say the O2 sensors on Bank 2 ( cylinders 2,4,6) on a V6 register a lean condition, then it would pop as Bank 2 running lean.

T-SB-0398-09 (December 23, 2009)

This service bulletin provides information on the proper identification of engine bank 1 and engine bank 2 for correct A/F sensor and oxygen sensor replacement.

This bulletin contains information that identifies engine bank 1 and engine
bank 2 on the following engines:

1AZ-FE, 2AZ-FE, 2AZ-FE (PZEV), 1GR-FE, 2GR-FE, 2JZ-GE, 1MZ-FE, 3MZ-FE, 1UR-FE, 3UR-FE, 2UZ-FE, 5VZ-FE, and 1ZZ-FE.

• Bank 1 (B1) refers to the bank that includes cylinder No. 1.
• Bank 2 (B2) refers to the bank opposite bank 1.
• Sensor 1 (S1) refers to the sensor that is located before the catalytic converters.
• Sensor 2 (S2) refers to the sensor that is located after the catalytic converters.

That said, It could be multiple things.

-Throttle Position Sensor (TPS) - https://axleaddict.com/auto-repair/How-Do-You-Know-if-a-Throttle-Position-Sensor-is-Bad
-Bad/Dirty MAF - https://www.tacomaworld.com/threads/maf-sensor-cleaning.26177/
-Fuel Pressure Regulator (FPR) - https://www.tacomaworld.com/threads/fuel-pressure-regulator.366132/
-Vacuum leak - This one most common
-Bad/clogged injector which is leaning out that cylinder. Usually accompanied by a missfire code but you never know. https://www.tacomaworld.com/threads/cleaning-testing-injectors.509459/
-Intake manifold leaks - same as vacuum leak
-Weak fuel pump - I had the same code for my 3RZ (4 cyl) and it ended up being the fuel pump. Fuel pump was working but not pushing enough volume of fuel so it was leaning the engine out. I did have some hesitation accompanying that.

1) First clean your MAF and TPS and test them.

2) Get yourself a can of flammable brake cleaner. Yes starting fluid works too but you get less per can. Let the engine run and start spraying around the intake manifold, vacuum lines, pcv valve, throttle body, etc... Do one quick 2 second spray in one area and wait 5-7 seconds. If you hear the engine idle up, you found the leak there somewhere. If not, go to the next area. Could be a hose, manifold, throttle body, etc...

3) If you don't find any leaks, move on to the FPR and general fuel pressure from the pump. Replace pump if weak.

4) Fuel Injectors. If you have a V6 what you could do is swap the 3 injectors from Bank 1 with the ones on Bank 2. Clear the code and drive it until the code pops up and see if the lean code changed to Bank 2. If so, then you have an injector leaning out on that bank. You can try to clean them up with some carb cleaner. You can hook up 12V to them to keep them open and replace O-rings. O-Rings are easy to find. If not you can replace the set. Rockauto.com has good options.

That would be how I would start. Let us know if you find anything.

 

You're thinking Sensor 1 and Sensor 2 for O2.

Bank 1 refers to Cylinders 1,3,5 and Bank 2 refers to Cylinders 2,4,6 on a V6.

So Bank 1 Sensor 2 would be Cylinders 1,3,5 side downstream (after cat) O2 sensor for example

 

I had the same problem and went through the process that Eon (above) put himself through. Same outcome. Had to go to a garage to find small vacuum leak. And that took care of the P0 171 problem. Good Luck

 

I've received some very good suggestions as to what to do next and I want to thank everybody for offering them! So far all I've done is clean the MAF sensor wires with CRC. Next in line is to squirt brake cleaner around the vacuum hoses and listen for RPM changes. If I can't find any, then I'll check the TPS.

I still am not sure how the ECU knows to trigger one code for Bank 1 and another for Bank 2. What sensors does it monitor to make that determination?

Just as a reminder, the P0171 code appeared about 200 miles after I completed replacing all six injectors, so who knows what I disturbed in the process. I don't think it is the fuel pressure regulator, I was very careful with that and only replaced the O-ring that seats it into the fuel rail.

Also, for the first time in 20 years I'm consistently getting over 20 mpg rather than 18 mpg. Sometimes when I drive a consistent 65 mph (ugh) I get as high as 24 mpg. Go figger.

 

Jesus maybe I need to service my fuel injectors lol, I'm lucky to get 15mpg on a good day. Added weight and roof rack doesn't help.

If nothing else it might be an issue with one of the new injectors you installed, but hopefully that isn't the case

 

I have to laugh, had the same dam problem with code and it was the vacuum hose leaking on my old boost gauge.

 

Since it was post injector change I bet it's a vacuum line or bad injector.

If it's a V6, then you have 4 O2 sensors.
2 on bank 1 and 2 on bank 2.

If the O2 sensors on bank 1 read that it's running lean then it will pop the code for Bank 1 and vise versa.

Banks are just sides of the engine. Cylinders 1,3,5 (bank 1) are on one side while 2,4,6 (bank 2) are on the other.

 

My truck smells like it is running lean, but i'm not throwing a check engine light. What tools do you guys use to read the codes for our trucks ?

 

Scanguage and Ultragauge are popular. I use an OBDII bluetooth plug-in that you can get off Amazon for ~$20 then connect to it via the Torque app on my phone. I can pull up all kinds of data including codes and pending codes.

 

Torque app is pretty good. But I invested in a Millenium 90 Pro by LAUNCH http://launchtechusa.com/millennium-90-pro/ from my MAC tools guy because I needed to be able to read/clear BMW, Porsche, Mercedes, Audi, Toyota, etc... SRS/ABS/Oil Light/Brake lights. Awesome tool. Lifetime updates and it even has instructions for diagnosis on codes you have never seen and how to say reset an oil service light manually vs automatically with the tool instead of googling. Worth every penny. Live data streams and all. "Full coverage, Domestic, Asian, European(Gas, Diesel, and Hybrid Vehicles 1996 and newer OBD2)"
It will read live O2 sensor read outs and air/fuel mix ratio
Cost me $300 bucks.

But that's like an enthusiast/side job tool. The step up from that is around $1500 for 2 way coms. such as activating particular solenoids and coding. Not in a position for that yet lol

 

3.4 has 2 sensors, bank 1 sensor 1 and bank 1 sensor 2, no bank 2 anything.

The lean code does not mean an O2 sensor is at fault. It means the sensor is still reporting a lean condition after ECU has added all the fuel it can in an attempt to correct the condition. The sensor "could" be reporting incorrect values but it's more likely just reporting the result of a fuel/air problem.
Time for diagnostics not guesswork.

 

@eon_blue is it this one

for Android Phones ONLY - Wireless Bluetooth Diagnostic OBD2 Scanner Car Code Reader and Scan Tool for All 1996 & Newer Vehicles ELM327 Compatible OBDII

https://www.amazon.com/BAFX-Product...s=gateway&sprefix=bluetooth+od,aps,140&sr=8-2

 

Yeah it's just like that, mine was a different brand but that will work. I actually wouldn't recommend the brand I got, because if I leave mine plugged into the ECU when I start the truck, it will trigger my ABS light on the dash for some weird reason. I have to plug it in after starting the truck to avoid that.

Anyway yeah, that one will work. Then all you need to do is download an app on your phone that connects to it, there are different apps depending on what phone you have. Torque is a popular one for Android. There's a free version and a ~$5 version that lets you do more.

 

@eon_blue

 

Hmm so the 3.4 V6 has 1 set of O2 sensors for 2 banks? Hmm fair enough. In that case it was ALWAYS show Bank 1 regardless of if it is a Bank 1 or 2 side lean condition. All the V6, V8, V10, and V12 I have worked on had 2 sets of O2s. One set for each bank. This narrowed down the side of the engine having the issue. Haven't worked on the 3.4 in these Tacos.

No guess work here chief. Read through the thread. I was merely explaining how a computer would determine which bank is lean answering the OP's question and posted some pretty detailed diagnostic tips....NEVER said that the O2 sensor was at fault.
https://www.tacomaworld.com/threads...-or-bank-2-running-lean.617454/#post-20991615

 

Not accusing you of anything or even replying to you, chief. Stating facts.

 

I assume you have a OBD 2 scanner? If so, go into your live data and look for your fuel trims. long & short readings -10 to +10 is normal but closer to 0 is better. If your getting high + like +20 something. that points to a vacuum leak. Low negative in my case was a bad MAF sensor