Alright, here's the deal.

Car's been running abnormally high for the past two days.

On day one, It was running at 70% (100% would be on the red H, 0% would be at startup after a night being cooled)
didn't think much of it, since it was pretty hot in Montreal for the past few days.

On day 2, Now the temperature kept fluctuating up and down from 70% to 90%.
Getting really alarmed, I decided to change the thermostat, thinking it was broken. And flushed the coolant at the same time.

Still no go.

Now the weird thing is, one day 1, I left it overnight, and at startup, the temperature is at 25%.
On day 2, left it sitting overnight again, and the temperature at startup is at 50% (sitting outside for 10 hours)

I'm thinking it's probably the sensor, but need more opinions on what it could be.
What are the symptoms of a bad coolant sensor?

Thanks in advance.
Fred

Well I'm sure it cooled down in those 10 hours. It's probably the sensor... or maybe a dodgy cluster?

probably the cluster circuit board.

I have the exact same problem- temp runs about 80 percent and stops before hitting the red- the car still runs fine and Ive driven the car a couple thousand miles with no problems....in fact I took it on a 200 mile road trip and it ran fantastic- I also heard it could be the coolant sender or sending unit may need to be replaced...btw I have a 98 ls400

Thanks.

I'm having it checked, but he says it's the cluster. In the other tread, I'm being told it's the temperature sensor.

Anyway I'll go with the temperature sensor first, then fix the cluster.

so what's the verdict? because i am experiencing the EXACT same symptoms right now with my UCF10. Anyone else have any useful information about getting the temperature down?

Im having mine fixed today- Im hoping its the sender unit- the car still runs fine

My temp gauge is still completely off. But the car runs as per normal.
Still didn't get the chance to look at anything, and had it looked by a lot of people.

Seems like either a sensor, or the cluster is defective.

Sounds like you might have an air bubble. Did you try bleeding the coolant system? The gauge and cluster might not be the problem. If you have air in the system it will make the gauge fluctuate like that. Mine did the same thing until I bled the system and then it was normal. Just a thought before you pay for a cluster you might not need.