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If it's not really about tuning or wiring. Then it belongs in here.
My 2000 Sienna has been good to me. Have driven her last 30K miles without any issue.
I drive about 30 miles a day.
On my way home from work, noticed that my temp gauge was climbing above the middle. By the time I made it up the hill, it was around 80-85% from the the top.
I left the car cool off for about 2 hours. I returned back to the car and saw that the expansion tank for the coolant was empty. I concluded that I must have a small coolant leak somewhere, and that I've finally lost enough coolant to cause me trouble. I filled up the expansion tank to the full line, and drove around for 10 mins.
It seemed fine for about 10 mins, but then the temperature gauge started climbing above the middle, and started overheating again.
I turned on the heat inside the cabin to max, and turned on the fans to max (while the temperature gauge was hover around 75%), but nothing but cold air was coming through the vents.
Next, I parked the car and opened the hood to expedite the cooling process. At this point, I noticed that the expansion tank was still filled to the full line.
Also, I noticed that the electric fans behind the radiators were not spinning, while the engine was overheating at 75%+.
I scanned the car with an OBD-2 scanner. Interestingly enough, right after the scan completed, the electric fans behind the radiators actually turned on for about a minute, and then turned off. The fans turning on was surprising, because it happened when the car was shut off. Car engine was off, but they key was in the turned position, right before cranking the starter.
What's going on here? Feels like it's a thermostat that's stuck closed ?
I had a similar problem with my Subaru, and that was a bad coolant temp sensor. The fact that you gauge was 75%, and the fans were not kicking on leads me to believe that.
I would not fool around with overheating, your heads can warp breaking the head gasket seal, replace the thermostat sensor first to ensure coolant is circulating the engine block.
Note: You can always remove your thermostat sensor, and put it in hot water 180 degrees+, it's just wax in there that expands to open the thermostat. If the thermostat is really bad, you should be able to visibly see it open or not.
If the ignition is still turned on, even without the engine running, heat soak can raise the temperature of the coolant to the point where the fans kick on. It's not that unusual if the coolant was already close to the switch temperature.
I do have some concern about the initial, apparent, lose of coolant - it had to go somewhere. Is it possible you have an air pocket somewhere that may be affecting the coolant readings for the gauge and ECU - assuming they use different senders, possibly in different places?
That turning the heater on to full would further indicate there's a problem with circulation - it's not uncommon for manufacturers to specify the heater is set to "hot" during filling and bleeding, in order to aid flushing out any air pockets. It's more an issue with heaters that use a valve to control the coolant in the matrix than those that adjust airflow, but still a good idea.
Oh, the sender could also be faulty.