PO171 trouble code in Toyota Vitz

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?