First time poster trying to get into cars. So my 2007 Toyota Corolla (with around 89k miles) has a Check engine light that reads error code P0171 (System too lean Bank 1). I was looking that up and the few causes are:

  • Intake/Vacuum leaks
  • Malfunctioning O2 sensor
  • Faulty/clogged Mass Airflow Sensor
  • leaking manifold gasket

The MIL turned on after I drove 300 miles from upstate NY to long island.

So my question is, how do I narrow it down to the problem?

I know there's listening to the engine idle or something but I'm not sure what I'm listening to and what the problem is.


  • Welcome to the site. There are a lot of possible causes for this. It looks like you have the scan tool; please post up the fuel trims that you are seeing with the engine running. This will enable us to point you in the right direction
    – Zaid
    Commented Jan 29, 2016 at 19:53
  • Am working on Toyota corolla 2005 and was given me p0171 and help Commented Sep 10, 2019 at 17:20
  • Has anyone seen it from running out of gas? And has it gone away?
    – A C
    Commented Jan 14, 2021 at 0:39

3 Answers 3


Using your scan tool watch fuel trims at idle, then while still stopped @ 3000 rpm. Fuel trims going to normal (anything below LTFT 10%) when revving point towards a vacuum leak or faulty MAF.

To check the AFR sensor pull a vacuum line with the engine running and it should go high lean. Then introduce fuel and it should go high rich. Toyota/Denso sensors run 3.33v ideally.

To verify MAF condition you do a pull while driving. Recording RPM and MAF g/s and use a volumetric efficiency calculator to verify MAF performance.

If vacuum leak visually inspect all hoses and the intake tube. Use MAF or brake parts cleaner and spray around the upper intake gasket.

You could do a fuel pressure test, but Toyota pumps rarely fail.

And when you do a repair make sure to clear fuel trim data. Failing to can lead to false positives.


P0171: oxygen sensor in bank 1 has detected a lean condition (too much oxygen in the exhaust). the side of the engine that has P0171 is one of the more common trouble codes. This code is triggered by the first downstream (front) O2 sensor.

Bank #1 refers to the side which has cylinder #1, In a 4 cylinder it is the cylinder in the front of the engine. The downstream O2 sensor monitors the condition of the catalytic convertor. If it senses O2-Houston we have a problem.

There could be a vacuum leak downstream of the MAF (Mass Air Flow)sensor. The MAF sensor is essentially is a sensor with a hot platinum wire and a thermistor. The idea is to maintain the same temperature in the wire. When the air flows over the wire cooling it. The current required to maintain the temperature is proportional to the air flow. Very accurate and responsive.

Possible cracked vacuum or PCV (Positive Crankcase Ventilation line/connection)

Faulty or stuck open PCV valve Failed or faulty oxygen sensor (bank 1, sensor 1) The up stream sensor tell the ECM how to keep the air fuel mixture optimum.

Sticking/plugged or failed fuel injector Low fuel pressure (possible plugged/dirty fuel filter)

Exhaust leak between engine and first oxygen sensor. The leak fools the sensor triggering the lean condition

The MAF (Mass Air Flow) Sensor is dirty or faulty. Can be cleaned with disc brake cleaner or anything that doesn't leave a residue. Ordinary throttle cleaners will leave a residue and render the sensor useless. Be very careful when handling a MAF sensor they are fragile and expensive.


I have seen this many times on corollas and it always turned out to be the intake manifold gasket. Its so thin and rubber. When I usually tske them out after remvoing the intake they are hard like plastic and brittle with some hairline cracks throughout gasket. This causes this code to pop up.

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