Check engine light on & OBD #P0171

For three months, the Check Engine Light has been coming on and off. The Onboard Diagnostic #P0171 showed when I went in to have a checkup. The following have been replaced: gas cap, mass air-flow sensor, and spark plugs. The told me that it was the mass air-flow sensor because the system is running too lean. The check engine light still comes on every 2-3 weeks. I feel like I’m having the wrong things fixed. Please give me your professional advice on this problem.
Experienced mechanics share their insights in answering this question :
P0171 for your car is "system too lean." The most common cause is a vacuum leak and it can come from a leaky hose or a leaky gasket. However, I did a quick search and found someone on the forums with your exact car and exact problem and his issue was a leaky fuel injector seal. You may want to try that first since those are cheap. Alternatively, a qualified professional from YourMechanic can come to your car’s location to inspect the Check Engine Light firsthand to have this corrected appropriately.

How to Identify and Fix Common car Problems ?

Our sources include academic articles, blog posts, and personal essays from experienced mechanics :

A P0171 code is set by your vehicle`s powertrain control module (PCM) when it detects that the air-to-fuel ratio of your vehicle is too lean. This means that the PCM is relaying that the engine is receiving too much air into the combustion chamber and not enough fuel.
Oftentimes, a P0171 means that your Toyota is having trouble with a dirty or faulty mass airflow (MAF) sensor, or a vacuum leak downstream of the MAF sensor (perhaps in the PCV line or in the valve itself). Of course, this code can also signify a bad O2 sensor in bank 1.
You probably know that the answer the questions is — no! A P0171 or P0174 lean code with an O2 sensor reading lean all the time mean — well, several things. The real problem may not be a bad O2 sensor, but possibly be an engine vacuum leak, low fuel pressure or dirty fuel injectors that are causing the engine run lean.
How Serious Is Code P0171? While it is common for a driver to only experience minor driveability problems when the P0171 trouble code is present, driving with this code can overheat the engine and cause internal engine damage.
Connecting to an OBD-II Port

You plug an OBD-II scanner tool into it and, in most cases, out pops a code that tells you what the problem is. The scanner also allows you to clear the vehicle`s internal code, which is what triggered the check-engine light. Do that, and the warning light will turn off.

What Causes Trouble Codes P0300, P0171 and P0507? The trouble codes you`ve mentioned are all indicative of a vacuum leak. We suspect somewhere around the engine there is more than likely a vacuum hose that is disconnected, broken or brittle.
Too much fuel and/or too little air results in the engine running rich setting a P0172 trouble code, while too little fuel and/or too much air results in the engine running lean resulting in a check engine light for the Nissan P0171 code.
How serious is the P0171 code? The P0171 code is a serious issue. If a vehicle has this code stored in its powertrain control module, the engine of that vehicle simply will not run well. If the engine does not maintain proper air/fuel ratio, there will be a major waste of fuel and a lack of power while driving.
Can bad spark plugs cause p0171? Not likely. A P0171 ( too lean, Bank 1 ) code is most often caused by a vacuum leak downstream from the MAF sensor or a malfunctioning MAF sensor. Bad spark plugs could actually cause the opposite code ( too rich, bank 1).
The most common fault codes associated with a clogged fuel injector are P0171 and P0174, which indicates the engine controller is seeing a lean condition.
Tip : A faulty or skewed coolant temperature sensor and/or air temperature sensor can cause certain systems to mis-calculate air/fuel mixtures, resulting in P0171, P0174 DTCs.
A plugged Fuel Filter or poorly functioning Fuel Pump can set the P0171 code. The computer hears (accurately) from the Oxygen Sensor that the Fuel Mixture is too lean so the computer keeps increasing the amount of fuel being delivered into the combustion chambers.
Steps to Reset an Engine Light

It will have a black cap and a negative (–) symbol. Using a wrench, loosen the nut on the negative battery terminal and pull the negative connector from the battery. Keep it disconnected for 30 to 60 seconds. Reconnect the negative cable and re-tighten it using your wrench.

Can I reset my ECU using an OBD2 scanner? No. An OBD2 scanner can only be used to detect error codes on your car. It cannot perform tasks like an ECU reset or any modifications on your car.
The easiest way to reset a check engine light is to disconnect the negative (black/-) battery cable and hold it to the side for 30 to 60 seconds. This will allow your internal diagnostics systems to reset.
Sometimes, the check engine light comes on to let you know that there`s a problem with the car`s internal diagnostic mechanism. That means the system could be “finding” a problem that doesn`t exist, or it could be that loose wire systems are creating a false positive in the system.
What does a DTC P0507 mean? The P0507 code indicates that the idle control system has an excessively high RPM – also called a `high idle`. When idling your car – which means keeping the vehicle`s engine running while it is not in motion – the Engine Control Unit (ECU) has to maintain a specific RPM range.
Faulty Air-Fuel Sensor or Oxygen Sensor

If this sensor is faulty (or the Oxygen sensor in some cars), it`s likely that the check engine light will be triggered by the P0172 code or a similar one.

Common Reason: Code P0172 is often related to a vacuum leak.
P0171 Causes

Some of the common causes for the DTC P0171 fault code include: A faulty fuel pressure regulator. A weak fuel pump. A clogged fuel filter.

Absolutely, a leaking exhaust manifold can cause those codes. Those codes do NOT mean the O2 sensors are bad, it means a lean condition has been detected.
When the P0171 code sets, the oxygen sensors are detecting too little oxygen in the exhaust (running “lean”) and the control module is adding more fuel than normal to sustain the proper air/fuel mixture. Rich Mixture = Too much fuel, not enough air. Lean Mixture = Too much air, not enough fuel.
When the P0171 code sets, the oxygen sensors are detecting too little oxygen in the exhaust (running “lean”) and the control module is adding more fuel than normal to sustain the proper air/fuel mixture. Rich Mixture = Too much fuel, not enough air. Lean Mixture = Too much air, not enough fuel.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

While driving on freeway truck goes into a limp mode, and the check engine light, abs light, parking break light, service brake
ANSWER : It is possible that you could be having an issue with your powertrain control module. This is somewhat of a common problem on many GM vehicles. I would recommend having your vehicle inspected by the dealership to determine if the powertrain control module is functioning properly. If not, a reflash of the powertrain control module may resolve the issue but often times it must be replaced.

Check engine light refuses to go off after engine replaced
ANSWER : Hello there, you have a long series of issues going on with your 2013 Hyundai Sonata. If you keep getting a repeating oil light on than there is likely a fault in that system.

Since this fault has lasted after the engine was replaced it could be a oil pressure sensor, wiring harness fault, ECU fault, oil pump, oil strainer clogged, or low oil pressure.

To diagnose this issue properly you need a complete vehicle inspection to start from square one and identify the root cause of all this.

2006 Ford Mustang has check engine light on
ANSWER : There are countless potential causes for your illuminated check engine light. The mechanic manually turned off the light, but it came back on because the problem has not been fixed. The first place to start is retrieval of the stored diagnostic trouble codes (DTCs). These codes serve as a starting point for any further diagnostics a mechanic may perform. Codes will be stored anytime the check engine light is on. Since the first mechanic was unable to fix your concern, I recommend giving the professionals at YourMechanic a call. We offer expert check engine light diagnosis and repair.

Car started to slow down and then the check engine light, battery, and engine oil light came on. What could cause this?
ANSWER : Hi there. It is possible that the battery cable came loose or the wires to the oil pressure sensor has came loose or failed. Check the battery cables and make sure that the terminals are tight and clean. Check the wires to the oil pressure sensor and make sure that they are connected and not broken. If everything looks good, then the oil pressure sensor may have failed. Plus, check the computer for codes and see what is stored. This will lead you to the part that could have failed. I recommend replacing the oil pressure sensor then checking to see if the lights will go out. If you need further assistance troubleshooting your vehicle, then seek out a technician, such as one from Your Mechanic, that can assist you with the warning lights that came on.

P2196 OBD-II Trouble Code. came and went in 24 hours. 4 starts.
ANSWER : If the mechanic tested the output (functioning) of the oxygen sensor, especially according to the protocol at this link and found that the sensor was non functioning at that time, what you might be experiencing is a "failing" oxygen sensor and so the latest instance of Code P2196 might be explained by that. A complexity though is Code P2196 has many causes other than a failing oxygen sensor including: a faulty fuel pressure regulator; MAF (Mass Air Flow) sensor; ECT(Engine Coolant Temperature) sensor or damaged ECT wiring; leaking or stuck open fuel injector or injectors; and spark plugs and/or coil pack. Basically, if the Code (P2196) recurs, the oxygen sensor output can be re-tested but the mechanic will hopefully be aware of all these other possible causes of that Code setting. If the Code recurs, you can request a check engine light diagnostic and a certified Mechanic, dispatched by YourMechanic right to your location, will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

p0011 code,check engine light one, changed bank 1 vvt solenoid and check engine light still on?
ANSWER : The P0011 code is typically caused by oil issues with the car. I would particularly make sure the engine oil level is not low. The VVT solenoid relies on proper oil levels to do their job. If they don’t have enough oil, they won’t work. You can have a mobile technician from YourMechanic visit your home or office to inspect the Check Engine Light for you. If you follow through with a repair with us you will be reimbursed for the cost of inspection.

Traction light/reduced engine power and check engine light coming on
ANSWER : Hello. From what you describe, it would appear that your vehicle may be experiencing some sort of intermittent wiring or sensor issue. If the problem occurred again after driving over a bump, then it may be possible that a damaged or loose wire is shorting or having contact issues. A common issue that can cause stalling is a faulty crank position sensor or wiring. I would carefully examine the engine for any loose or damaged wires. If the Check Engine Light comes back on, I would recommend having the trouble codes scanned before resetting the computer.

Check Engine Light comes on after starting pickup
ANSWER : Hello there, thank you for asking about your 1992 Isuzu Pickup. A professional will have the correct equipment to read OBD1 codes. Retrieving the diagnostic trouble code(s) is the first place to start when diagnosing your vehicle. Increasing RPMs changes engine load, which can in turn affect many different sensors and components, causing the Check Engine Light to illuminate. The only way to narrow down the options and find the culprit is through in person diagnosis with an OEM compatible scan tool. A certified technician from YourMechanic can retrieve the OBD codes at your home or office.