P0171 Lean is a very common fault code on Toyota’s. In this article I am going to be discussing the 1.8, 2.4 4 cylinder engines in the Toyota Corolla, Camry, Rav4 and Matrix along with the V6 3.5 liter engine used in the Camry, Avalon, Solara and Sienna van. The last vehicle on this page is the Highlander. I did not cover the Toyota Tacoma or Tundra. However, keep in mind, the theory and operation is still the same when it comes to a lean condition on the V8 engine and all other internal combustion engines with this similar Toyota fuel system. First is a brief overview of this code. A P0171 Diagnostic trouble code indicates a lean condition on bank one. Bank 1 is the side of the engine or “bank” that the number 1 cylinder is on. Bank 2 is simply the other bank. If it is a 4 cylinder engine, there is only one bank. Below is a list of the more common Toyota engines you will see. (note: this is not a complete list)
1.5 (1ZN-FE) 4 cylinder
1.6 liter (4A-FE) 4 cylinder
1.8 liter (7A-FE) 4 cylinder
1.8 liter (1ZZ-FE) 4 cylinder
1.8 liter (2ZR-FAE) 4 cylinder
1.8 liter (2ZR-FE) 4 cylinder
2.2 liter (5SE-FE) 4 cylinder
2.4 liter (2AZ-FE) 4 cylinder
2.7 liter (1AR-FE) 4 cylinder
3.0 liter (1MZ-FE) V6 engine
3.3 liter (3mz-FE) V6 engine
3.5 liter (2GR-FE) V6 engine
4.0 liter (1GR-FE) V6 engine
4.6 liter (1UR-FE) V8 engine
5.7 liter (3UR-FE) V8 engine
P0171 Fuel system indicates lean Too Lean (Bank 1)
System: PCM/Emissions/fuel system
According to the Manufacture, here are the possible causes for this code.
Vehicle driven low on fuel or until it ran out of fuel
HO2S or Air fuel ratio sensor
Fuel injectors, dirty or pressure regulator has failed
Fuel control sensor is out of calibration (Map, MAF or ECT)
Base engine “mechanical” fault affecting one or more cylinders
Air leaks in the exhaust manifold and in the exhaust pipes
Fuel delivery system (low fuel pressure due to a fautly fuel pump or restricted fuel filter)
2004 Toyota Corolla LE 1.8L, 4 cylinder engine VIN R.
The customers concern is the check engine light on with a Diagnostic trouble code stored in the computer of P0171 indicating the fuel trims are too high and the engine is running lean. Based on my experience, this condition can be and usually is (but not always) associated with a performance or rough running condition. For example, the engine will bog down or hesitate while accelerating or it will stall or surge at lower engine rpms. However, this is not always the case. The problem may be on a much smaller scale where the computer on the Toyota Corolla can compensate for the lean condition and the driver may never feel a hiccup while driving. I will go over some of the more common ones I have seen on a Toyota Corolla and even the Toyota Matix as they use the same 4 cylinder engine.
First, I want to say this is a very common fault code on this engine. However, there three common causes for this lean condition and P0171 code and I will go over each of them in detail.
1. Mass air flow sensor.
The MAF or Mass air flow sensor measures the amount of air coming into the engine. This measurement must be precise so that the computer can match up the amount of fuel with the amount of air coming into the engine. In addition the changes of the amount of air coming into the engine or driving conditions change very fast as the driver accelerates and lets off of the gas pedal so the response time must be quick , fast and accurate. Usually what happens is the sensing element on the mass air flow sensor gets dirty and it cannot measure the amount of air coming into the engine properly. This will skew the signal of the Toyota’s on board computer and will result in a severe lean condition and will bog or hesitate under a load or while trying to accelerate. Sometimes you can clean the sensor element but most of the time replacement is necessary. Based on my experience, the mass air flow sensing element gets dirty because of a dirty air filter that has not been serviced or replaced regularly. So it is always a good idea to replace the air filter anytime the mass air flow sensor gets replaced. The O.E. part number for the mass air flow sensor on this Toyota Corolla is 2220422010 or 222040D030. You can get it here. Keep in mind, that even if the sensing element on the MAF looks good and clean, this does not mean the mass air flow sensor is producing a correct signal. The sensor signal may still be incorrect or “skewed” even if the sensing element looks clean and free of debris.
Testing the mass air flow sensor
To test the mass air flow sensor using a scanner reading live data. When the mass air flow sensor is faulty it will have low grams per second readings. The grams per second at idle should be about 1.8 – 2.2 and at 2500 RPM and no load, the readings should be about 7.2. Low calculated load readings during a wide-open throttle acceleration can also indicate the mass air flow sensor is faulty. The calculated load reading should increase to 85% or higher on a wide open throttle acceleration. You can also watch the fuel trims under a load too. High positive numbers at idle and normal numbers while accelerating indicates a vacuum leak. Normal numbers at idle and high positive numbers under a load indicate a faulty mass air flow sensor. Good fuel trim numbers are 0% or as close to 0% as possible. Anything up to plus or minus 7% is considered OK. Plus or minus 7% to 20% is considered “bad” or “excessive”. Another thing you will want to check is the air filter itself. Not only is should be clean, but it should be stock. If there is an aftermarket type of air filter, remove it and see if the fuel trims remain normal. Again, a dirty air filter can and will eventually cause the mass air flow sensor to get contaminated and go bad.
To test the mass air flow sensor using a voltmeter, you can go here for the manufacture specifications and get the correct wiring diagram for this circuit. This will ensure you are on the “signal” wire.
If you would like some help diagnosing this condition or need assistance with your vehicle, you can speak with a Toyota expert here.
2. Air/vacuum leak
The second most common cause of this P0171 on Toyota Corolla’s would be an air/vacuum leak. On this particular engine, it is the intake manifold gasket that goes bad and leaks engine vacuum resulting in this code. This condition will only affect the engine at idle because the engine vacuum is at it’s highest point at idle. So it it has a vacuum leak, it will show up at idle or lower engine rpms and usually cause a surging or high idle condition if the vacuum leak is large enough. This will not affect the way the engine accelerates. This is so common Toyota released a TSB or Technican Service Bulletin about this condition. It is Toyota TSB EG045-07. This covers 2003 to 2008 Corolla’s and Matrix’s with a 1ZZ–FE 1.8 liter engine. This TSB goes into detail about the diagnostic procedure to pinpoint and fix this code (to confirm there is a leak where the intake manifold and cylinder head meet between each intake runner) and explains that there is a new updated or intake manifold gasket to fix this condition. The old part number is 171710D020 or 17171–22030. The new and improved or updated part number for the intake manifold gasket is 17171–22060. Technical tip: A leaking intake manifold gasket will cause this lean code to set during the warm up process in most cases.
3. Front oxygen sensor or air fuel ratio sensor
The third and final common cause for this P0171 lean code on this Corolla with a 4 cylinder 1ZZ–FE 1.8 liter engine is the front oxygen sensor or air fuel ratio sensor. However, it is important to note, that this sensor should only be changed after replacing the mass air flow sensor or ruling it out as a possible cause and fixing any air or vacuum leaks under the hood first. Short of doing these two things first will result in the check engine light coming back on with the same lean code. The part number for the front oxygen sensor/AFR sensor is 8946513030. Here it is on Amazon.
Note: While this code is common on 1998 – 2006 Toyota Corolla’s with a 1.8 liter engine, there are some older Corolla’s that still flag this code but do not use a mass air flow sensor. Instead, they use a MAP sensor. For example, a 1998 Toyota Corolla uses a MAP sensor intead of a MAF sensor. The O.E. part number for the MAP sensor on this vehicle is 8942002030. While the MAP sensor may go bad, it is not very common. Even if it did go bad, I have not seen it flag a lean code. If you see a lean code on this year Corolla, it is usually caused by a faulty oxygen sensor in front of the converter or an air/vacuum leak.
Note: Also, verify the evaporative purge solenoid is not stuck open or being grounded all the time by the Engine Control Module (ECM). Toyota issued a Technical Service Bulletin (TSB) EG041-07 related to this concern. This TSB talkes about the ECM constantly grounding the purge solenoid which may cause this Lean P0171 code. Based on my experience, if this was the case, it will also flag large and small Evaporative Emission codes.
Note: If you only have a P0133 diagnostic trouble code stored in your computer, there is another TSB that was released by Toyota for this code on Corolla’s and Matrix’s. The PCM Logic been modified to improve this condition. In other words. the fix is to recalibrate the main engine computer.
2007 Toyota Camry LE 2.4 Liter engine 2AZFE 4 cylinder Vin V
1. Mass air flow sensor.
I have seen this vehicle with a number of trouble codes including the P0171 lean code stored in the computer. Again with the most common cause of a lean code on this engine would be the mass air flow sensor provided there are no air or vacuum leaks under the hood. You can test the mass air flow sensor using a scanner or a voltmeter. Testing it with a scanner or scan tool that can read live data is recommend. You can watch the fuel trims to see if they go positive under a load or acceleration indicating the sensor element on the MAF is not able to read the correct amount of air coming into the engine. Again, normal fuel trims are plus or minus 7%. Also, you can watch the engine load under wide open throttle as it should go about 85 or 90%. This is provided you have a scanner with the update or refresh rate fast enough you can see live data as fast as it changes. A scanner that has a slow update rate would be hard to use to diagnose the mass air flow sensor. In that case, I would recommend to use a voltmeter while backprobing the signal wire at the sensor while the engine is running so you can get a live reading. The O.E. part number for the mass air flow sensor on this Toyota Camry with a 2.4 liter engine is is 2220431020. You can get it here. This vehicle also has a history is flagging P2195, P0138, P0031 along with the P0171 lean code. Toyota Technical Service bulletin 0074-08 addresses these codes. According to that TSB, they also make a special socket (part number 09224-00010) to remove and replace the air fuel ratio sensor.
Here is a picture of a mass air flow sensor on a 2008 Toyota Rav4. This is the same engine, sensor and set up as the 07 Toyota Camry with a 2.4 liter engine.
2. Air/vacuum leak
The second most common cause of a lean code on this vehicle is a faulty brake booster. The brake booster is vacuum assisted and works off of engine vacuum. If the brake booster goes bad and leaks engine vacuum, it will affect the fuel trims and the computer will flag this P0171 lean code. Technical Tip: While watching the fuel trims on your scanner, engine fully warmed up, step on the brake pedal. If the fuel trims go up or start to climb positive, this indicates a leaking brake booster. For this year vehicle U.S. built, there are two different options. With Stability Control or without Stability Control. With Stability Control the O.E. part number is 4461007131. Without Stability Control the O.E. part number is 4461006302.
3. Front oxygen sensor or air fuel ratio sensor
Another very common Diagnostic trouble code for this Camry we see a lot is a P2238. The definition for this code is “Air Fuel Sensor (Bank 1 Sensor 1) Pumping Current Signal Low“. On this vehicle, this particular code is caused by the air fuel ratio sensor. Toyota released a TSB for this code. It is EG032-04 and indicates there is an updated updated air fuel ratio sensor and an updated program for the Powertrain Control Module to fix this P2238 code. The O.E. part number for the front oxygen sensor or air fuel ratio sensor for this vehicle is 8946706030. According to this TSB, the updated part number for the air fuel ratio on this vehicle is 89467-06070. You can get it here or here. This TSB also indicates that this repair is covered for 96 months or 80,000 miles
Note: This engine has a major oil consumption issue. However, I have not found that this is related to or that it has caused any of these codes above.
Note: This is the same 2.4 liter 4 cylinder engine they use (around this year) in the Toyota Rav4’s They also have an option for a V6 3.5 liter engine. More of this below.
2005 – 2007 Avalon and 2007 – 2008 Toyota Camry 3.5L, V6, 2GRFE, 24V
This particular Toyota Camry using the 3.5 V6 engine has a history of flagging this trouble code P0171 lean code as well. There are at least two related TSB’s for this code. The first one is Toyota TSB 0114-08 talks about this code along with other diagnostic trouble codes stored in the computer of P2195, P2197, P0138, P0158, P0174, P0031, or P0051. According to this TSB, the fix for this these codes is to replace BOTH air fuel ratio sensors and the right hand side exhaust manifold. The updated part numbers for the air fuel ratio sensors are 89467-07030 for the left side and 89467-07040 for the right side. The updated part number for the right hand side exhaust manifold is 17140-0P041. Affected vehicles, 2005 – 2007 model year Avalon and 2007 – 2008 model year Camry vehicles with 2GR-FE engines.
The TSB is (EG050-05) DTC P0171, P0174, P2195, OR P2197 related to the air fuel ratio sensor. The Air Fuel (A/F) sensor manufacturing process has been improved to correct this condition. The updated part number for bank 1 air fuel ratio sensor is 89467–07020. The updated part number for the air fuel ratio sensor for bank 2 is 89467–07010
Vehicle Application: 2008 RX350 3.5 and 2008 Sienna 3.5
1. Fuel pump
On these two vehicles, the Customer Concern is usually the check engine light will illuminate and Diagnostic Trouble Codes of P0171 (bank 1) and P0174 (bank 2) are stored in the computer. If you have access to a scanner that can read live data, you may notice the fuel trims are high at idle and high while at steady cruise. While this condition could be caused by a faulty mass air flow sensor, on these vehicles this is usually caused by a failed or weak fuel pump. When testing the fuel pressure on this engine, the specifications are 44.1-49.7 PSI. It is important to note, and after the engine is shut off, the fuel pressure should stay above 21 PSI for 5 minutes. If the fuel pressure is lower than the specifications or if the fuel pressure leak down below 21 psi after the engine is turned off, the fuel pump has likely failed and will need to be replaced. On the Sienna the O.E. part number is 7702008040. The O.E. part number for the Lexus RX350 (Canada built) is 232200P030 and (Japan built) is 2322031180
2. Fuel injectors
The fuel injectors are also a common concern for these codes on this 3.5 liter V6 engine. The best way to check them is to do an injector balance test. You can even ohm them out, but they usually don’t go bad this way. The tips or ends usually plug up restricting fuel flow resulting in a lean condition. If you have one injector that you suspect is bad or plugged up causing the engine to misfiring, you can swap that fuel injector with another fuel injector from a different cylinder to see if the misfire code or misfire condition moves along with the fuel injector. The O.E. part number for the fuel injectors on both the Lexus RX350 and Sienna is 232090P040. Here are they are on Amazon.
Note: The VCS light is a very common light to come on as well with these lean codes. Usually it will be C1201 and indicates there is an engine control problem. However, it will go away when the lean codes are repaired.
Note: If the fuel pressure is OK, check fuel quality and fuel/alcohol percentage.
Note: Ensure that the correct NGK or Nippondenso spark plugs are installed.
Vehicle Application: 2007 Sienna 3.5 and 2008 Toyota Sienna 3.5
P2195 “Air Fuel Sensor (Bank 1 Sensor 1) Signal Stuck Lean” is another common cause you will see on this vehicle. It may or may not be associated with the P0171 lean code. While the Air Fuel Ratio (A/F) sensor is the most common cause for this code, here is a complete list of possible causes for this P2195 code.
Air fuel ratio sensor has failed.
Air or vacuum leaks
Air fuel ratio circuit is open or shorted to ground
Check for a fuel delivery component problems
Check for air leaks in the exhaust manifold and in the exhaust pipes
PCM has failed
Testing for a diagnostic Trouble Code P2195
Unplug the Air Fuel Ratio sensor connector and turn the ignition switch on. Check for voltage on the wires coming from the Engine Control Module. All 4 wires should read voltage; 12 volts for the heater voltage input, 3.3 and 3.0 volts for the Air Fuel Ratio sensor signal inputs and 2.4-2.8 volts for the heater control feedback. You can swap the Air Fuel Ratio sensor from side to side and check to see if the code now sets for the other bank (bank 2). If so, the sensor is faulty and will need to be replaced. The O.E. part numbers for the Air Fuel Ratio/oxygen sensors are listed below.
Upper front… 8946708070
Upper rear… 8946708040
Lower front… 8946508090
Lower rear… 2 Wheel Drive 8946508070
Lower rear… All Wheel Drive 8946508080
Vehicle application: 2004 Toyota Highlander with the (2AZ–FE) V6
There are two TSB’s that we see for this vehicle. The first one is TSB EG032-04 for DTC P0031 Oxygen (A/F) Sensor Heater Control Circuit Low (Bank 1 Sensor 1) AND/OR P2238 – Oxygen Sensor Pumping Current Circuit Low (For A/F Sensor) (Bank 1 Sensor 1). The A/F sensor manufacturing process and the Engine Control Module logic have been modified to improve these conditions. The updated air fuel ratio sensor to fix these two codes are 89467–48070. Remember, this is only for bank 1, not bank 2.
The second TSB we see for for this vehicle is TSB EG004-05. It covers DTC P0031 (Oxygen Sensor Heater Control Circuit Low [Bank 1, Sensor 1]), P0051 (Oxygen Sensor Heater Control Circuit Low [Bank 2, Sensor 1]), P2238 (Oxygen Sensor Pumping Current Circuit Low (for A/F Sensor), AND/OR P2241 (Oxygen Sensor Pumping Current Circuit Low (for A/F Sensor) [Bank 2, Sensor 1]).
According to this TSB, the fix is to replace both front Air fuel Ratio sensors and recalibrate or update the PCM. The new updated part numbers for the air fuel ratio for bank 1 is 89467–48060 and for bank 2 is 89467–48050.
While these last two TSB’s do not specifically talk about the P0171 lean code, if the air fuel ratio sensor goes bad, it can definately cause a lean condition.