It'd come 3 months ago, have erased it with ELM327.

Fine for few months and it's back.
the difference is it's coming back every time I start the car even after I erase it.

No performance drop, but I'm a slow driver so I might be missing it.

​​​​​​My guts telling me the temp under the hood has been hotter than before, but not sure.

Here's the screenshot of ELM327 when idle.
https://ibb.co/mABNPk
Do you see anything strange? how should I fix it?

 

Possible causes Faulty front heated oxygen sensor Ignition misfiring Faulty fuel injectors Exhaust gas leaks Incorrect fuel pressure Faulty Mass Air Flow (MAF) sensor

Read more: https://www.engine-codes.com/p0172.html

Does it run smooth or feel like it has a miss?

 

It runs very smooth. except the light coming up no performance drop at all

 

How is your fuel mileage? Does the exhaust smell rich? It may be as simple as a bad O2 sensor or bad fuel pressure sensor.

 

I don't see any difference in fuel mileage, at least on trip computer.

the strange thing is it went away by itself.. didn't come back after 2 hours of drive.
the day it popped up was a really hot day.. any clue on reason why?

 

Yes, it went off itself, and then came back again .

smells fuel from the exhaust. the color is okay (not black)

checked ELM327 and I only see one O2 Sensor. (bank 2)
The others are N/A.

Is it the problem? This car has 2 O2 Sensors right?

 

The most common cause for this problem is a disconnected or cracked vacuum hose causing the computor to command a richer fuel mix to compensate for the lean condition. Right under the intake and hard to get to is a vacuum hose that fails alot, I believe it go's to the PCV system.

HTH,

 

decided to clean maf sensor, located it. but I can't get to remove it.. do you know any good write up?