My Toyota Vitz (Yaris) 2007 randomly gives PO171 (System Too Lean) when at idle. My mechanic has done following fixes so far, but problem still persists.

  • Replaced oxygen sensors.
  • Replaced fuel filter.
  • Cleaned injectors, spark plugs.

Also, mechanic checked for a vacuum leak by spraying propane, but he couldn't find a vacuum leak. But, he didn't do a smoke test.

Following are some scanner readings at the last trouble code:

  • Short Term Fuel Trim: -0.01%
  • Long Term Fuel Trim: 31.24%
  • MAP: 60 kPa
  • RPM: 823
  • Vehicle speed: 0

It is seen that LTFT is pretty high but STFT is OK.

I checked live data and the STFT and STFT are at similar ranges at idle. When I increase the RPM, both LTFT and STFT comes to around 10%.

When I searched for the problem, I could find that if LTFT is high at idle and low at high RPM, it is probably a vacuum leak. But, my situation is different.

Also, the air pressure also varies arround 40 and 60 kPa. Could this be due to a problem with the MAF?

What else could be the cause to this issue?

  • Has the throttle body and MAF been cleaned? Has the ECU/PCM been reset to cause it to relearn the throttle curves (see this questions: mechanics.stackexchange.com/q/23407/4939)?
    – CharlieRB
    Mar 20, 2018 at 12:27
  • @CharlieRB Throttle body was cleaned, but MAF was not. Actually This vehicle does not have a MAF but a MAP sensor. Mechanic didn't clean it either. Also, the battery was disconnected during the repair work, so it should have re-set the PCM. After the repair, the driving didn't feel like it was before for few days. I think it should be due to PCM re-learning.
    – Softcode
    Mar 20, 2018 at 13:45
  • I would recommend performing the relearn procedure again. Just because the battery was disconnected, it may not have forced a relearn and causing the computer to run on old values with different components.
    – CharlieRB
    Mar 20, 2018 at 15:37
  • Have you tested the fuel pressure regulator or back-brobed the Injectors for an accurate oscilloscope reading.. a smoke test can be made with a simple bike pump, a bottle and a hose to the location,...then stick charcoal in the bottle and pump :)
    – user38183
    Aug 13, 2019 at 2:55

2 Answers 2


If the throttle body, MAF/MAP was cleaned, or other fuel/emission components replaced, the throttle relearn needs to be done.

  1. Disconnect the negative cable from the battery (cover the post with a cloth to avoid shorting).
  2. Disconnect the positive cable from the battery (cover the post with a cloth to avoid shorting).
  3. Touch the positive and negative cables together for about 30 seconds. This drains voltage and resets the PCM.
  4. Connect the positive battery cable.
  5. Connect the negative battery cable.
  6. Turn the key to run but not start for about 10 seconds, then start the engine.
  7. Let it idle until it reaches normal operating temperature.
  8. Take it for a drive with normal acceleration and full stops at intersections.

Note: the engine might idle rough after the reset until it relearns the idle curves. It should smooth out after a few minutes.

Here is a YouTube video that shows how to do this - Throttle body relearn.


Here's another possible cause, that I discovered while preparing to clean the Mass Air Flow (MAF) sensor using MAF cleaner spray; big as day, saw a big crack in the hose from the MAF body to the engine.

enter image description here

I was getting a P0171 intermittently, but it went away, but the hesitating/surging continued.

This picture is from a 2009 Chevy Aveo so if you have a Toyota, ... this won't look familiar, but usually the MAF body is close to or connected to the air filter box.

  • 1
    Thank you so much. This is exactly what happened to my car. Not easy to discover until I pull the hose around.
    – Shiji.J
    Nov 3, 2020 at 3:26
  • What was wild, was I had the car at the Chevy dealer a few weeks before I discovered this, and they totally missed it. I got a new hose online, for around $60; the dealer wanted around $150 -- the dealer had it in stock, so apparently this is a common issue. Nov 3, 2020 at 14:33
  • Mine was because the dealer bent it too hard while doing the throttle body cleaning. They replaced it for free next day.
    – Shiji.J
    Nov 14, 2020 at 23:47

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