Got P0116 code for Toyota Sienna

I can get you started. The coolant temperature sensor will be located near the thermostat housing. You can find that by following the upper radiator hose to it. If you have a temperature gauge on the dashboard, that will usually have a different sensor with one wire. The one you need for the Engine Computer will have two wires in its connector.

Use an inexpensive digital voltmeter to measure the voltages on those two wires. That must be done by back-probing through the back of the connector next to each wire, and the connector must not be unplugged. One wire should have very near 0.2 volts. The other one must have between 0.5 and 4.5 volts. Typically at 60 degrees you can expect to find around 2 - 3 volts. The ignition switch must be turned on too for those measurements.

If you find 0 volts on both wires, there is either a break in the 5.0 volt feed wire or you're not making a good connection with the meter probes. In that case, unplug the connector and measure the voltages right on the terminals. One of them should have 5.0 volts.

If you find the full 5.0 volts on only one wire when the plug is still connected, and the other wire has 0.0 or 0.2 volts, there is a break between those two terminals. The sensor itself could be defective, and I can tell you how to verify that, but that would be very uncommon. More likely one of the terminals is stretched or corroded and not making a good connection.

If you find 5.0 volts on both wires, the ground wire is open. You'll have to search for the break in that wire.

Hello
ECT sensor is a thermistor sensor which monitors water temperature. DTC is set when ECT sensor signal value is out of range during engine operation. Possible causes are:
†Defective ECT sensor.
†Defective cooling system.
It is possible with 119k on it that your thermostat is going bad. Have you noticed any change in your engine temperature range? It may be slow opening or closing. Also, it may be your ECT that is going bad too.
Here is a pic of your thermostat and the location of the ECT. Me, I would change them both. They are not that much and again, you do have 119k on the car. Also, according to the info, you are supposed to be replacing the coolant every 30,000 miles or 24 months.
Also, I notice they have a TSB out on this also but your car is out of the warranty time, so I would have to assume it isn’t the engine control at this point.
M.I.L. "ON" DTC P0116
TECHNICAL SERVICE BULLETIN
Reference Number(s): EG043-04, Date of Issue: October 1, 2004
TOYOTA: 2000 Sienna
SECTION: ENGINE
Related Ref Number(s): EG043-04
ARTICLE BEGINNING
INTRODUCTION
Some 2000 model year Sienna vehicles may experience a M.I.L. Light "ON" condition with DTC P0116 (Engine Coolant Temperature Circuit Range / Performance Problem) after being cold started in temperatures between 35°F and 50°F (2°C and 10°C), and the car is at idle for at least 2 minutes after starting. The Engine Control Module (ECM) (SAE term: Powertrain Control Module/PCM) logic has been revised to correct this condition.
APPLICABLE VEHICLES
2000 model year Sienna vehicles.
So †my recommendation is to change your thermostat, I would recommend the failsafe type, change the ECT and get your system flushed if you haven’t.

The thermostat is a mechanical device that is buried inside a metal housing. The electrical connectors in the photos are for the two coolant temperature sensors. The one with the single yellow wire is only for the gauge on the dash. You want the one with the two-wire connector. That is for the Engine Computer. If that sensor or its wiring have a problem, it could incorrectly tell the computer that the engine is colder than it actually is. That will result in too much fuel going in and black smoke from the tail pipe.

This doesn't sound like a thermostat problem. The code refers to an electrical problem with the sensor. Those sensors give very little trouble except on Ford products, so I would look first for corrosion on the terminals in the electrical connector. If you do decide to replace the sensor, wait for the engine to cool down so there won't be any pressure in the cooling system, then just unscrew it and pop the new one in. Very little coolant will run out.

If the thermostat housing is up on top, you would lose about 1 - 2 quarts of coolant when you remove it. Draining some first won't help because there will be some trapped in the upper radiator hose that won't drain out. It will wash onto the top of the engine when the housing is unbolted.

I shy away from opening the petcock in the radiator too to drain coolant because there have been a bunch in past years that do not seal again. Part of the plastic tank breaks leading to a drip about once per minute. The only fix for that is a new radiator. Instead, if you have to drain the coolant, it's better to slowly remove the lower radiator hose.

There are a few different symptoms to a leaking head gasket. The most noticeable one is white smoke from the tail pipe and the coolant level will go down over a short period of time. Another one is bubbles in the reservoir even before the engine is warmed up and there will be no steam with those bubbles. The coolant might even be cold yet.

One test that can determine if the head gasket is allowing combustion gases to leak into the cooling system involves drawing air from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow. You will need to have a mechanic perform that test. It only takes a few minutes.

Based on your previous comments, I don't think you have a head gasket problem. It sounds more like an electrical problem related to the coolant temperature sensor.

Sorry to take so long to reply. The site isn't working on my main computer so I have to borrow a different one.