Coolant fan is now always on

Thermostats go bad in 3 ways... They stick open. They stick closed. They stick closed until too warm and then pop open and stick open until too cold. In option 1, your engine will typically never get fully warm while moving. In option 2, your engine will typically overheat. In option 3, it will bounce, as said, between too hot and too cold, typically over 5-10 minutes.

More likely, it's either a fan controller, sensor or sending unit OR the fan is on continuously because your van is borderline overheating. There are multiple different temperature sensors and sending units on the van. You NEED to find out if the van is overheating ASAP, if you don't want to toast the engine. If it is, but the gauge doesn't indicate it, you have a coolant leak AND a bad sensor or sending unit. I believe on the Sienna that there are three different sensors for temp. One feeds the fan controller, one feeds the gauge and one feeds the ECU. If the gauge and the ECU temp are in agreement and you have plenty of coolant, it's probably the fan or fan controller.

If the temp dial is reading higher than normal, my guess is either faulty thermostat or coolant isn't circulating properly for some other reason (low coolant, etc).

As I've mentioned in a previous post, temp gauges "lie" to you. To avoid people panicking over small but normal variations, most car temp gauges will always go to the exact same spot in the normal range even as the temp varies over some predefined range.

On my 2013 Honda Pilot, if I watched the actual temp with a scan tool, when it got somewhere around 160 °F I could watch the gauge slowly but surely "pop" into position at the normal spot. It would stay there as the temp rose to 175 °F. When towing, the temp got as high as 225 °F and the gauge still hadn't budged, though at stops I could hear both fans blowing on high to help cool the engine.

All this to say, if your temp gauge has moved higher than normal, and your fans are blowing all the time, your engine is getting a lot hotter than it should be getting (unless you were towing heavy things and didn't tell us that). I agree with what's been posted. If you have a scan tool, use that. At the very least check your coolant reservoir tank. If that's got coolant when the engine is cold, then you're probably not low on coolant. In that case you need a new thermostat.

What year van do you have, and what engine? Do you do DIY repairs, or take it to a shop?