Trouble Code P0125 on 2000 Sienna

[lifesheba]

My 2000 Sienna has 143K miles and is suffering code P0125. The Van drives fine other than the check engine light is on. I have done the following repairs but nothing helps.
1. Change Engine Coolant Temperature Sensor 89422-35010
2. Change No.2 ECT Switch 89428-30130
3. Check No.1 ECT Switch; works fine
4. Check Thermostat; works fine
5. Flush and change engine coolant/anti-freeze
I used to have P0155 and P0125 came up at the same time a while ago. I replaced O2 sensor (Bank2, sensor1). It was OK for a little while then the P0125 shown up. It was intermitten in the beginning when I drove with cold engine in the winter. It was fine if I warm up/idle the engine for 5-10 minutes then drove. Now it shows up everytime I drive; very consistent with no other code.
I disconnected the battery then idled the engine for ~15 minutes yesterday with the heater in full blast (both front & rear); driving around without triggering the check engine light. The code reader pulled P0150 with no other code. I erased the code and let the Van sit overnight.
I drive the Van again and P0125 shows up again. Erase it many times but it always come back right away. Your tips will be much appreciated.
Thanks,