In my Toyota Vitz 2007, PO171 (System Too Lean) trouble code comes up randomly (check engine indicator lights up). Earlier, the trouble code came up rarely and only when the vehicle is idle for some time (usually in a heavy traffic), but now the frequency of the trouble code has increased and comes up when vehicle is not idle.

My mechanic has done following fixes so far, but the problem hasn't resolved.

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

When scanned with a OBD scanner, it can be seen that the long term fuel trim is high, which should have caused the trouble code. Following are data at a trouble code.

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

Also, there is another issue in the vehicle. Sometimes as soon as vehicle is started, it refuses to move when I step on the accelerator. The engine RPM doesn't increase. If I keep on pressing, the engine may increase the RPM and car starts to move slowly. If I turn off the engine and start again, the issue usually gets fixed. When vehicle is running, I see no difference in pickup. This issue sometimes comes up when the vehicle is stopped while going uphill and tried to move uphill again.

Car also has an issue with a dashboard indicator. I have asked this in a separate question.

Dashboard indicator comes up with key exclamation sign

What could be the reason for this? Could this be due to a problem with the ECU?

  • What year/engine do you have, and what is your approximate elevation? 60kpa MAP seems high for idle.
    – Milison
    Apr 24, 2018 at 16:20
  • @Milison, the year is 2007. The elevation is almost at sea level. But, as I could remember, other instances of the trouble code had lesser MAP values (about 45kPa to 55 kPa). I couldn't check the MAP value without starting the engine to check whether the reading matches atmospheric pressure to see whether there is an issue with the MAP sensor.
    – Softcode
    Apr 24, 2018 at 16:26

2 Answers 2


A had the same code in a Honda Civic. It turned out to be a tear in the intake rubber hose. The mechanic almost missed it. It was rubber fatigue caused the heat of the engine and the cycles of pulling backing and forth when accelerating. It was causing a vacuum leak. I recommend taking a close look there.


Your Long Term Fuel Trim at idle is 31.24 % .Try doing this-Set your Rpm to 2500 rpm and see if your fuel trim is improving, ie coming down. Next go to 3500 rpm and see what the trims are at this rpm. Check and record trim values at the idle, 2500 and 3500 rpm settings as stated earlier. +/- 10% long term fuel trim is considered normal on most systems.Also if your upstream sensor is fixed rich, the computer will severely lean out the fuel mixture.

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