I've replaced the thermostat on my van along with a new seal, new upper and lower radiator hose and fresh 50/50 coolant. Drove it today and noticed the engine temperature gauge was reading very high. I mean getting near the 260 mark. I have no leaks anywhere and the coolant level is fine. I wondering if the coolant sensor has failed?

That's scary!!
Did you get all the air out? I have had to remove the heater hose connection at the rear of the intake manifold (while refilling) to get that last pesky air pocket out.
How about the rear heat and hoses? Some of the high mounted systems could trap air.
If it was really that warm your clutch fan should have been engaged and raising a fuss/very loud.
IIRC the gauge sensor and the computer use separate sensors and the gauge sensor is in the left side head. But don't trust me on that.......

Check that your radiator core tubes haven't ballooned, or you can still see through the radiator.

With air in the system you can see the gauge fluctuate quite a bit as air then water hit the gauge.

If your system lost a lot of coolant, then some goes through the system and hits a cylinder wall the resulting steam and expansion can rush through the system and balloon the radiator tubes. No air flow no cooling. (i've experienced this one)

After that check your fan belt and fan and make sure no air.

Also might do a pressure test on the system if you inadvertently ran it hot prior to the replacement and caused other issues.

Or you put the thermostat in facing the wrong direction.

The new thermostat could be defective as well. If that is the only thing you changed and the overheating started immediately it points to the thermostat, or there is still an air bubble trapped somewhere.

Also what temp thermostat did you install? Because I run my van only in summer I run a low temp version. Air trap has happened to me in the past.

Hey JD,

Marcela, JO, Jason, and Catfish right on info input all good! "Tiny Bubbles," Sing it!!

As an aside, I'd add look for bubbles in coolant flow, cap off, engine warm. A blown head gasket may pressurize the cooling system as well. This will force the coolant out of the cooling system. Absent a pressure test, A blown head gasket may be indicated by the presence of "Tiny Bubbles" in the coolant flow. Seen it, but IDTS in this case tho. My money's on trapped air.

Those Tees that come in the Prestone FlushnFill kits are pretty handy for bleeding air out from the cooling system. Just install that puppy in an uppermost part of the heater hose, Presto! ne...

Update: Drove 50 miles today and the reading was normal. Sat for about 1.5 hrs and drove back home the 50 miles and the reading was 260 for 30 miles then went to halfway mark the rest of the way home. I watched and listened for trouble signs and the only thing I saw was the needle doing a dance a few times. going back and forth from 125 - 200. This makes me think the problem is electrical. Again, there are no leaks anywhere on the engine or under the engine. the exhaust is normal/clear. I added enough coolant to top off the radiator before i left.

I'll bet you're happy you don't have to pull that thermostat again:)

I ran the engine with the heat on and radiator cap off for a good 20 minutes and I think i've got all the air out of the system. The temp gauge stayed normal (around the first mark). thanks for the tip on the trapped air.