2004 Toyota Sienna 3.3 L P0128 Coolant temp below thermostat regulating temp

First off, what is the temperature reading on the scan tool? This code is usually a faulty thermostat. Does it get to operating temperature?

I'm with Jose, does the engine get up to operating temperature?

For me, as long as the cooling system is full, the water pump/coolant is free flowing, the fan isn't stuck on, and the ECT is telling the truth, then you're done.

To date, I've never seen a P0128 that didn't get fixed with a thermostat. I still go through the checks, of course, but the OE diagnostics for that code are usually spot on.

Most coolant sensors have a ressistance of about 3000 to 4000 ohms cold which drops to 200 to 500 ohms approaching running temperature --- sensors vary between manufacturers but those are pretty universal on the ones I encounter of Euro boxes.

Then I would do an unplugged resistance test on the sensor, and test the refrence voltage supply on the harness. I those are both Okay, plug it back in and back pin the plug and check the voltages as the engine heats up.

These numbers are for a Prius but should be pretty universal for Toyota

20 degrees C = 3550 ohms + or - 500 ohms

80 degrees C = 335 ohms + or - 35 ohms

90 degrees C = 240 ohms + or - 30 ohms

It's counterintuitive, right? :silly: The best explanation I can offer is that the ECT doesn't get an accurate reading if it's not immersed in coolant, and will show colder than actual engine temp. This kind of false code may also depend on the engine, and where the ECT lives in the system.

Thermostats and fans control the band of normal temperatures the engine runs at typically on a modern engine this should be 88c to 115c after a couple of miles driving The thermostat controls the low end of this and the fan the upper end. 115c seems high but typically 50-50 coolant at radiator pressure boils about 125c .
On modern cars the temperature gauge on the dash will remain on "N" between about 75c and 115c

99.999% of engines circulate coolant from the bottom of the radiator into the water pump up through the cylinder block then through the cylinder head to the thermostat then to radiator top hose back to the radiator or when the thermostat is closed through the by-pass hose and heater.

Most cars built between about 1955 and 2010 use a simple wax thermostat -- the thermostat blocks coolant flow out of the top of the engine going through the radiator until it reaches the start to open temperature of the thermostat on modern cars this typically 87 to 91c, the thermostat will not fully open until about 3c degrees above this. Before the thermostat fully opens coolant is diverted through is a by-pass hose which also supplies the heater that allows continuous coolant circulation from the top of the engine back to the water pump inlet. Wax thermostats have the advantage that they should fail safe --- when they go wrong they open at a lower temperature.

A lot of more modern engines are doing techie things with thermostat for example moving the thermostat down to the bottom of the engine, using electric pre-heaters on the thermostat, spring loaded thermostats that open at lower temperatures when the rpm is raised (called a PRT) On a lot of recent cars it can take a bit of figuring out which way the coolant is flowing but essentially it is still the same.