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Engine efficiency is highest when the air-fuel mixture ratio is kept at 14.7 parts air to 1 part fuel. Too much fuel and/or too little air cause the engine to run richly, setting the P0172 difficulty code, while too little fuel and/or too much air cause the engine to run lean, setting the P0171 Toyota code and turning on the check engine light. A vacuum leak, which adds more air to the air-fuel combination, or a poor fuel system, which does not add enough fuel to the mixture, can both contribute to the lean state. The powertrain control module (PCM) attempts to correct the lean state by adding extra fuel to the mixture in an effort to maintain the correct 14.7:1 ratio in order to keep the engine operating properly. The Toyota P0171 code is set off when these modifications are made to much of a degree.
In This Article...
What causes the Toyota P0171 error code?
A P0171 code frequently denotes a problem with your Toyota’s mass airflow (MAF) sensor, a vacuum leak downstream of the MAF sensor, or both (perhaps in the PCV line or in the valve itself). Naturally, this signal may also indicate a malfunctioning O2 sensor in bank 1.
A P0171 code can also occasionally (but not always) be set off by a clogged fuel injector, a dirty fuel filter that lowers fuel pressure, or even an exhaust leak that occurs before the first O2 sensor. Isn’t diagnosing engine issue codes enjoyable?
- Cleaning the MAF sensor and the lines leading to and from it is typically the best course of action in response to a P017. This usually makes the problem go away.
- Make sure to check all of the lines, including the vacuum and PCV hoses, after cleaning the MAF sensor. Check the lines for kinks, cracks, etc., and replace them as necessary. Additionally, make sure to inspect all of the hoses and connections on the air intake and look for leaks in the intake manifold.
- Last but not least, replacing your air and fuel filters is always a good idea, especially if they are about to expire.
Replace the O2 sensor on bank 1 if cleaning the MAF sensor and checking the lines don’t resolve the issue. For detailed model-specific information regarding its precise placement, consult your Toyota’s manual. If that doesn’t work, it’s probably time to seek expert assistance.
On the Toyota Parts website, you may find this article with a complete list of issue codes.
Despite our best efforts to ensure the quality and accuracy of this trouble code summary, we cannot be held liable for any errors or omissions. Before doing any repairs to your car, please make sure to consult a repair manual for that year and model or call a licensed, professional automotive technician.
How do I resolve the P0171 system Too lean Bank 1 issue?
If no other error codes are present, a mechanic can identify the P0171 code by using a vacuum gauge to look for vacuum leaks in the engine. A hissing sound caused by a vacuum leak is typically only discernible while the engine is off.
A fuel pressure gauge will also be used to examine the MAF sensor (mass air flow) and the fuel pressure sensor.
The vacuum and fuel pressure of the engine must be within the manufacturer’s limits in order to maintain a correct air/fuel ratio. One of these two checks will probably reveal the source of this error number.
After the mechanic has run these two tests and the issue is still not resolved, a sensor is most likely to blame.
The mechanic would then perform tests using the manufacturer’s instructions on the mass airflow sensor and the oxygen sensors.
The powertrain control module is most likely the source of the issue if all of these tests reveal no problems.
Can I operate a vehicle with P0171?
Take the reading carefully whenever your check engine light is on. This regulation does not apply to the P0171 code. You shouldn’t drive your car very far, if at all, when the engine stores this code.
When your engine is running lean, it lacks the fuel it needs to function properly. You run the danger of the car losing its ability to accelerate quickly enough to keep up with oncoming traffic or to climb a steep incline. The vehicle can possibly overheat and break beyond repair.
P0171 codes are more likely to appear in turbocharged vehicles whose boost has been increased. Generally speaking, more air results in more power, but to maintain the ratio, more fuel must be added.
Your car is not using its fuel effectively if code P0171 is present, at the very least. Every time you drive, you’ll waste gas and money.
Can a P0171 code be caused by a filthy air filter?
The air filter or air cleaner component should be examined first. A lean running state occurs when the air flow is restricted or impeded by a clogged or extremely dirty filter.
The P0171 error code can also be caused by a clogged fuel filter and a dirty or malfunctioning MAF sensor in addition to a filthy air filter. Brake cleaner should be used to clean the MAF to help isolate or rule out this possibility.
Another possible cause of the error is a PCV valve that is broken or stuck. In some circumstances, a stuck or malfunctioning fuel injector also sets off the fault code. A clogged or dirty fuel filter may also be to fault for the issue code if the motor is not receiving enough fuel.
However, in some circumstances it can also indicate the presence of a vacuum leak in the PCV or MAF sensor connections. The bad news is that detecting a vacuum leak without the aid of specialized equipment like a smoke tester can be challenging. Sometimes the problem is due to an O2 sensor that isn’t working properly.
How to fix the P0171 code?
You must check for and remedy vacuum leaks, fix low fuel pressure, or replace any malfunctioning sensors that could result in a lean mixture on bank 1 in order to resolve the P0171 error code.
What causes P0171 code?
The vacuum or intake leaks, low fuel pressure, broken PCV valves, or malfunctioning MAF sensors are the most typical P0171 code reasons. Other things can potentially trigger the P0171 code.
What does P0171 mean?
The O2 sensor detected a lean mixture on bank 1 according to the P0171 error code. If the combination is outside of the sensor’s correction range of +-15%, the p0171 error code will be recorded.
How to clear code P0171?
You require an OBD2 scanner in order to erase or reset the P0171 error code. Keep in mind that simply resetting the code won’t address the root cause of the issue, and it will probably return.
What might result in a P0171 code?
- Fuel rail: Verify the pressure in the fuel rail. The P0171 code may appear if the pressure isn’t high enough, which is an indication that your car isn’t getting enough fuel.
- Check to see if the fuel injectors, filters, or pump are clogged or obstructed, as this would prevent the engine from receiving the proper amount of fuel.
Bad Oxygen Sensors
O2 sensors keep track of any fuel or unburned material that enters the exhaust. A P0171 code may be set off if an oxygen sensor isn’t communicating with the computer in the right way.
How much does repairing P0171 cost?
A diagnostic check for this code will be necessary; depending on how long it takes to identify the problem, the cost will be around $100. The average cost per repair will be quite variable because there are numerous causes for why an engine runs low. With one of the straightforward fixes suggested above, you might be lucky. A blocked gasoline filter costs $100, while a broken vacuum hose that is sucking in air costs roughly $50. An oxygen sensor or mass air flow sensor, however, can cost between $400 and $500. A damaged gasoline pump might cost anything from $600 to $800.
Bank 1 is too lean in what ways?
The front oxygen sensor determines whether the air-fuel combination entering the engine is lean (too much air and not enough fuel) or rich (the right quantity of air and fuel). It does this by measuring the amount of oxygen in the exhaust fumes (too much fuel and too little air).
The PCM modifies the fuel delivery accordingly to maintain the air/fuel ratio at 14.7/1, which is the ideal level (14.7 parts of air to 1 part of fuel).
The PCM continuously adds additional fuel when the oxygen sensor detects an excessive amount of oxygen, believing that the air/fuel mixture is lean. The PCM lowers the fuel level when there is insufficient oxygen in the exhaust (rich air/fuel mixture).
Can a malfunctioning fuel pump cause P0171?
The P0171 error code may be caused by a clogged fuel filter or an ineffective fuel pump. The computer continuously increases the amount of fuel given to the combustion chambers after receiving an accurate signal from the oxygen sensor that the fuel mixture is too lean.
Does a P0171 error code result from a defective O2 sensor?
The check engine light will turn on when the ECM/PCM needs to supply too much gasoline as a result of information from the O2 sensor and receives the problem code P0171. This error code could indicate that the mixture contains either too little fuel or too much oxygen, depending on the oxygen sensor.
Can lean conditions be caused by spark plugs?
The spark plugs don’t affect the fuel mixture on their own. They are just required to set off an ignition so that the fuel mixture can burn. However, faulty spark plugs have a particular ability to deceive oxygen sensors in contemporary vehicles, resulting in a lean situation.
Unburned fuel is one of the main signs of faulty spark plugs. Although incomplete combustion, not a rich situation, is the underlying cause of this problem, the o2 sensor may interpret the problem as a lean condition since the fuel is being released into the exhaust.
As the o2 sensors take some time to attain operating temperatures and communicate the required information, you might not immediately detect the symptoms of a lean fuel mixture.
Can a p0171 code be caused by an exhaust leak?
Yes, having a lean code for one bank eliminates things that apply to both institutions. This eliminates MAF and Fuel. Individual DTCs are set by O2s for heater failure or volt high or low (internal short to power or ground).
A p0171 would probably not be caused by an exhaust leak upstream of the O2, as that would require ‘clean’ air being drawn in via the venturi effect.
methods for diagnosis
1. Check the coolant system’s pressure.
2. With the engine running, unscrew the radiator cap (degas bottle cap on 04’s) and check the exhaust.
3. Take out the spark plugs. There will be steam-cleaned white plugs on the problematic cyl.
4. Eliminate HO2S; steam cleaning will also be performed.
Those are merely alternative approaches to the problem; they are not steps. Take the heads to a machine shop for cleaning and a leak test even if you do tear it apart and discover a faulty gasket.