What is code P0125 on 1998 Toyota Sienna?

P0125 (Closed Loop Fuel Control Insufficient Coolant Temp).

SAE DTC P0125=Coolant Temperature Insufficient for Closed Loop Operation.
Manufacturer DTC P1135 (for Toyota) means A/F Sensor Heater Circuit Malfunction Bank 1 Sensor 1 (Set when the heater operates, heater current exceeds 8A or heater current of 0.25A or less).
The A/F sensor contains a ECU-controlled heater. At start-up, the heater helps warm the A/F sensors to quickly operating temperature. With minimal exhaust gas flow, the heater keeps the A/F sensor from cooling down.

Checking Resistance.
a. Disconnect the A/F sensor connector.
b. Using an ohmmeter measure the resistance between terminals +B and HT. Resistance: 0.8 - 1.4 ohms at 20°C (68°F). If the resistance is not as specified, replace the sensor.
Torque: 44 Nm (440 kgf.cm, 31 ft.lbf)
c. Reconnect the A/F sensor connector.
(http://www.alflash.narod.ru/O2/79062351.gif )
Quotes.
P0125 set if after the engine is warmed up, A/F Sensor Output* does not change when conditions (a), (b), (c), and (d) continue for at least 1.5 min.:
a. Engine speed: 1,500 rpm or more
b. Vehicle speed: 25 - 62 mph (40 -100 km/h)
c. Throttle valve is not fully closed
d. 140 sec. or more after starting engine
*Output values changes inside ECM only

Occassionally vehicle misses or hestitates when you accellerate.

I suggest checking for a blown afr fuse in the engine compartment block. If ok check the sensors for proper operation. If they stay at 3.3v or if 02 sensors, they flat line replace them.

Malfunction with the #1 O2 sensors on both the right and left bank so my recommendation would be to replace both the bank 1,sensor 1 and bank 2 sensor 1, they are both located in the exhaust manifold.

Air fuel sensor is probably the problem; that is what the code po125 is really foR

Problem deals with bank 1 only(cly1,3,&5). Look up TSB for air/fuel part #'s on 3.0. Rear update # differ from front(near radiator). If # of both same or rear has RAV #, which can happen at parts,the A/F sensor will cause this kind of problem.