Saw the CEL coming on, brought it to workshop to check. Using OBDII, error code is P0134, and it states that the O2 sensor is faulty.
Just sharing these pic if you guys are interested to know how the sensor looks like:

This is the new one to replace:

Total cost: S$246 including labour.
It would be cheaper if you buy the part from stockist yourself and bring to workshop to change. For me, too lazy to run around so change it on the spot. After changing, my car became slightly more responsive and it picks up better. Now monitoring the FC.
>> Just ran 2 full tanks, milleage hit 493 for 1st tank, 505 for 2nd tank before yellow lights came on. Slight improvement compared to my previous 430 - 450km.
Just sharing this piece of info with you guys. :)