My 2005 Vios, P0125 error code & how i solved the problem
I copied & published my article previously posted here
Hi All,
Let me share my experience in solving the P0125 Error Code
What is the problem
P0125 error code – Insufficient Coolant Temperature For Closed Loop Fuel
Please read the problems detailed here > http://bit.ly/1CCAgM
What have i replaced (which did not solved the problem)
After numerous visits to the mechanic (only one 
), he advised to changed below items:
1) ECT (Engine Coolant Temperature) Sensor
2) Thermostat
3) Flushing the Coolant
I even let my engine running without Thermostat (which i hesitated to do) after been advised by the mechanic. Yet, the problem still arise.
What I ALMOST did
The mechanic has run out of idea to solve this problem. He EVEN asked me to proceed to do a TOP overhaul to my engine and replacing the gasket. Then see whether the problem will crop up again.
Luckily i didn’t budge to his advise.
How do i solved the problem
I’ve posted couple of questions in this forum and i received some feedbacks which is very in the quest of solving this problem.
The main reply that leads to this problem solving is by Jefflwc http://bit.ly/1fv9OR in http://bit.ly/Ujoev
I also received feedbacks from Deuce & ToyoT1R here http://bit.ly/2I3vtD
I went to the mechanic again to test my O2 sensor. It seems to be ‘OK’. Only for a while. After using the car for 2-3 days, the ‘check engine’ icon lits again. I went to the mechanic AGAIN, he asked me to do the TOP overhaul. I said no and he erased the DTC and the icon light is off.
Deep in my heart i believe, the O2 sensor is defective for whatever reason(s) there are. I’ve made many cross-reading in the web, many people have the same problem and solved by replacing the sensor.
Last week was a trying time for me. I’ve the ‘check engine’ icon lighted twice. Once on Tuesday, and lits off when i starts the engine the next morning. And lits again when i was travelling to KLIA (scary meh , with your family onboard)
So i went around to buy the sensor, unfortunately i can’t get any. I sourced out from Pong (nice guy), he didn’t have any stock. So i went around, i found a spare part shop which opened on Sunday. I ordered (given the right parts number) and on Monday, i pick it up. At this moment, the check engine is still lits, mind you.
On monday, after pick it up, i went to the nearest workshop, to get the O2 sensor fix. In an instance, the check engine icon lits off. Alhamdulillah. Till now, everything is fine.
What if your car has this error code? What do you need to do?
Go straight to check your O2 sensor. It might getting defective overtime. http://bit.ly/1fv9OR
You need to have the OBD II computer to check it (your mechanic will have it). Check the voltage fluctuation. If there is none, it’s due for change.
You can ask him to check the temperature reading (received from the ECT sensor) before the fan kicks in. If it is normal (mine is around 96 degree (if i can recall it right), ECT sensor is still OK.
What is the unlikely problem device? Your thermostat. If it stuck open, the water will still flow. If it stuck close, you will have different problem…hehe. Different error code will raise up. Make sure your coolant is sufficient too.
What i learned from this experience?
1) Your mechanic can advised you heaven & earth. You decide.
2) Consider to use this forum to solve your problem & share back your experience with the forummer. Sharing is caring.
Last, this is the picture of the defective O2 sensor and the newly bought one. 
p/s: Make sure you order the sensor with the correct Parts Number. You have been warned.
O2 Sensor
New O2 Sensor
