My name is Shawn and I have a 2001 toyota corolla. I just replaced the front oxygen sensor,coolant temp sensor and new plugs. But still my engine light is coming on with a P0171 engine to lean code. What else could be wrong or cause this? The only thing I have left to replace is the mass air flow sensor. Would this be recomended as the next step as to remedy this? Please respond with an accurate answer as possible thank you

P0171 means Fuel Trim System Lean (Bank 1)

The MAF (Mass Air Flow) Sensor is dirty or faulty.

Note: The use of "oiled" air filters (K&N, etc.) can cause the MAF to become dirty if the filter is over-oiled

There could be a vacuum leak downstream of the MAF sensor.

If your FT is leaning it that much, I would suspect an O2 sensor or it's circuit is the problem. When that O2 sensor went to 2V, I think you found your problem. The O2S is telling the PCM that the fuel ratio is way rich, so the PCM is assigning a way lean fuel trim.

Replacing the Mass Air Flow Sensor should do the trick.

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I would check for exhaust leaks, restricted injector, or low fuel pressure this can cause this code also. I would NOT through a several hundred dollar MAF sensor at it.

DTC P0171 is set when fuel trim is driven rich beyond a certain value. Possible causes are:
Exhaust system leak
Improper fuel pressure
Fuel injector restricted
Defective heated oxygen sensor No. 1
Defective MAF sensor
Defective ECT sensor
Vehicle ran out of fuel

This is out of Mitchell On demand

Hello, i am assuming you have replaced the fuel filter. The items you replaced certainly could have caused you problems. Also be sure to check for vacuum leaks, most of the time you can hear a hissing noise at idle if you listen closely. Correct any vacuum leaks found. The mass airflow sensor is probably faulty though, have seen this many times with this code. Hope this info helped you! Have a great day!

Assuming that you have disconnected the negative cable for a while after O2 and Coolant sensor replacement, I would have good clean all gunk built up at throttle body to see it it help
Replacing MAF would not improve the situation if you don't have a change with gas millage and occasional hesitation for a split second.
A lot of people have this code keeps showing up after a few days since they replace the same parts like you. It turns out to be a problem with the A/F sensor and computer: both have to be replaced. These are expensive, if your car is under 100K, check with dealer, they might cover the parts and you only pay the labor.
Some people told me that If they have a better millage since the engine light on, they would put up with the light and enjoy the gas saving. What do you think?

I would investigate the EGR valve before I replacing the MAF unit. Typically when the MAF is malfunctioning it will trip its own code and the car would idle rough and run like **** if it started at all,. Good luck

This would be the next logical step. The air flow sensor is what actually controls the air flow to the intake manifold. When you press the gas pedal you are actually operating the air flow to the engine. Then the fuel injectors follow with more fuel.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/craig_3fa289bf857b1a3c

Check the freeze frame data with the scanner. If the fuel trim is high teens or in the 20s then its the MAF that the problem. If fuel trim is normal, 2-5 then its the O2 sensor.As you have already replaced the O2 just check the MAF SENSOR whether its normal or not.

first try this, Just take out the MAF, get a cotton swap and some rubbing alcohol and clean up all the black gummy stuff off the wire sensor.

MAF-
Buy "Electrickleen" at the parts store.
Remove the MAF sensor, WHILE COLD, and blast the wire inside with cleaner.
The "hotwire" gets gummed up and changes the resistance of the wire so you get a bad input to the ecu and it trys to lean out for a rich condition that doesnt exsist.When it cant adjust anymore it trips the po171 code.
DTC P0171 is set when fuel trim is driven rich beyond a certain value. Possible causes are:
1. Exhaust system leak.
2. Improper fuel pressure.
3. Fuel injector restricted.
4. Defective heated oxygen sensor No. 1.
5. Defective Manifold Absolute Pressure (MAP) sensor.
6. Defective Engine Coolant Temperature (ECT) sensor.


When the air fuel ratio feedback is stable after the engine is at operating temperature and the fuel trim has reached its limit of correction to the rich side. (2 trip logic).

Feedback from the HO 2 sensor influences short-term fuel trim and short-term fuel trim influences long trim fuel trim. Short-term values are temporary and not stored when the ignition key is turned off. Long-term values are stored in memory because they are part of the basic injection duration. Long term values affect injection duration in closed and open loop because they are used to calculate basic injection duration. It is important to remember that the actual fuel trim will be the opposite of the DTC. A system too lean, DTC P0171 will mean the ECM is making a + or rich correction.
read (and let me know)above data and Freeze Frame by using (example, generic) Scan Tool on # mode 2, etc.
Slowly accelerate the engine in the increments shown and record voltage,
grams/sec of MAF and SFT, LFT, Injection Pulse, Fuel Pressure, etc.