Question about Toyota Corolla

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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

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  • cave_shawn Oct 08, 2009

    I have replaced the coolant temp sensor and the front xoygen sensor but still getting this engine lean code also at Idle or when the engine in under load it seems like its not getting enough power to climb the grade it kicks into overdrive several time before it gets to the top of the grade I would like a for sure opinion on replacing this mass air flow sensor

  • cave_shawn Oct 08, 2009

    I guess I haven't made myself clear enough about this problem!! These suggestions that I have recived I have already tried. I have cleaned the MAF but still no positive result. everything else is new concerning the fule syestem. Except the MAF. This is becoming very frustrating and expencive to remedy. The only thing I have left to change is the MAF. So should I take the gamble and change the MAF and hope for the best?

  • cave_shawn Oct 08, 2009

    This year of toyota corolla 2001 don't have an egr valve!! The mass air flow sensor replaces that type of sensor. At least anyway my car doesn't have 1

  • cave_shawn Oct 08, 2009

    I had another person tell me today that the converter may be pluged but my car seems to have a fair amount of power just the lagging on the accelerator while idling and when the engine is under load! It sounds like the engine is chocking for air thats why I want an opinion on replacing the MAF thats the only option seems like I have left

  • cave_shawn Oct 09, 2009

    I understand what you are saying but the thing is , is that the fuel milage is not the greatest. Seems like if I reset the ECM it gives me better fule milage for a while until the engine becomes under load such as going uphill! Then I start to hear whats calls a spark knock soon after the engine light comes back on! This is why I would assume it would be the MAF sensor since it controls the air fule mixture! I have cleaned the trotle body with carb cleaner and also tried cleaning the MAF with cleaner but with no positive results

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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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Posted on Oct 09, 2009

  • Onifade Adewunmi
    Onifade Adewunmi Oct 09, 2009

    Replacing the Mass Air Flow Sensor should do the trick.

    Good luck.


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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

Posted on Oct 08, 2009

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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!

Posted on Oct 07, 2009

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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?

Posted on Oct 08, 2009

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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

Posted on Oct 08, 2009

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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.

Posted on Oct 08, 2009

  • Craig Butler
    Craig Butler Oct 08, 2009

    If the air flow sensor is not correct, the engine control unit gets a mis read on the air/fuel ratio.

  • Craig Butler
    Craig Butler Oct 08, 2009

    One more question...how does it actually run? Does it seem to have plenty of fuel? It's not running sluggish? Plenty of pep like it has plenty of gas?

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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.


Posted on Oct 08, 2009

  • INDIRA KUMAR
    INDIRA KUMAR Oct 08, 2009

    Do you need additional details over this?

  • INDIRA KUMAR
    INDIRA KUMAR Oct 08, 2009

    If you have tried every thing what i have posted then there is no other go take a gamble and change the MAF sensor mostly as per my knowledge it must fix your problem.


    If satisfied please rate and accept my solution for all the help i have provided

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P 0420 code on toyota corolla2004


Hi there:
A code P0420 may mean that one or more of the following has happened:
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  • A damaged or failed oxygen sensor (HO2S)
  • Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
  • The engine coolant temperature sensor is not working properly
  • Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
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About the possible solutions, some suggested steps for troubleshooting include:
  • Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
  • Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
  • Inspect the downstream heated oxygen sensor (HO2S), replace if necessary
  • Replace the catalytic converter
Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily.
We strongly recommend that if you need to replace the catalytic converter that you replace it with an OEM unit. Second choice would be a high-quality replacement part. There are many stories in our forums where folks replaced the cat with a cheaper aftermarket one only to have the code return shortly thereafter.

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Corolla, 1999, P0125


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Possible causes- Low engine coolant level
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DTC P0125 OBD-II - Insufficient Coolant Temperature For Closed Loop Fuel Control
This means that the engine coolant temperature (ECT) sensor indicates that the engine has not reached the required temperature level to enter closed-loop operation within a specified amount of time after starting the engine. You will likely not notice any drivability problems.

A code P0125 may mean that one or more of the following has happened:
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This code refers to the front oxygen sensor on Bank 1. The heated circuit in the oxygen sensor decreases time needed to enter closed loop. As the O2 heater reaches operating temperature, the oxygen sensor responds by switching according to oxygen content of the exhaust surrounding it. The ECM tracks how long it takes for the oxygen sensor to begin switching. It the ECM determines (based on coolant temp) that too much time elapsed before the oxygen sensor began operating properly, it will set P0135. You will likely notice poor fuel economy the illumination of the MIL.

A code P0135 may mean that one or more of the following has happened:
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Hope this helps; also keep in mind that your feedback is important and I'll appreciate your time and consideration if you leave some testimonial comment about this answer.

Thank you for using Fixya, and have a nice day.

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My 2003 Toyota Solara will not pass inpection because the EVAP and catalyst monitors won't become ready. I have driven over 400 miles on highway and another 90 around town and have done a Toyota recommened...


Both of these monitors won't run until the oxygen sensor monitor has run. The oxygen sensor monitor won't run if an Engine Coolant Temperature (ECT) sensor Diagnostic Trouble Code (DTC) is stored. So, did you replace the errant coolant temp sensor? I take it you did and reset the codes. But you need to check the codes again and see if there are any that returned or are pending. A pending misfire code will not allow the oxygen sensor monitor to run. (Pending codes will not turn on the check engine light.)

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MPG excessive .10mpg 2001 Chevy silverado auto. 4.8 eng.I have new air filter fuel filter fuel pump OE tires,new oil 10w/30 pplugs look god as do the wires .No OBD lights truck runs great why the low MPG...


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Not just a faulty catalytic converter will trigger this code. Many other issues can trigger this code.

  • Leaded fuel was used where unleaded was called for
  • An oxygen sensor is not reading (functioning) properly
  • The engine coolant temperature sensor is not working properly
  • Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
  • Delayed spark timing
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  • Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
  • Inspect the downstream heated oxygen sensor (HO2)
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Mass Air Flow sensor! Just remove your air filter cover. Go to Auto Zone and purchase some solvent. Spray past the air filter and it should correct the problem. If not, replace the Mass air-flow sensor. This is commonly mistaken for an oxygen sensor. Mine did this and I followed these steps and it fixed the light. BTW, the light stayed on for 5 years before I fixed it. Never hurt anything. It's normally something to do with emissions. Dummy lights. :)

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