99 Accord EX model: The thermometer often goes to H, when I turn the heater on, it goes back down. This happens when stuck in traffic...however if I don't turn the heater on the thermometer will not go back down....now, if the car has been idling for a while, it will fluctuate. I've been told that it's not overheating and the purchase of a new guage cluster will fix the problem....is this accurate? Here's a little more background...about a year ago I was loosing coolant like crazy, they've replaced the radiator, the hose to the radiator, the radiator cap (2 times), and the thermostat. Please help.

 

first thing first, are you sure your car is not overheating?

you can lower the temperture if you blow hot air without recircluation, i had to do that when my head gasket was toasted.

things to check

1)FAN FAN FAN, see if both fans kicks in in local traffic, you will not see much problem on the hwy due to the amount of air flow

2)jumpy needle could means air in the system, you had abunch of work on the cooling system but did you bleed everything afterward?

3)before you bust out the old gauge, check the voltage of your water temperture, i think it's a yellow with green wires in the bay, it should read around 6v(at normal driving temp.) if you see the same jumpy pattern in your voltage, it's not your gauge then

sidenote, it's not very common for integrated circuit board gauge cluster to loose ground, it could be it will affect all the ground, not just the temperture. this is why i want you to check all the stuff before replaceing the gauge. like i said, the older bolt and cooper sheet style gauge cluster can loose ground due to corrotion and vibration but it's not very likey for intergrated board like the one you have now

 

If you do replace the gauge, make sure you ground yourself out. Static electricity can destroy electrical components!

 

It sure as hell sounds like an overheating problem.

I agree with iam7head... It really, really, really sounds like one of your fans isn't coming on, and that the temp fluctuation is actually a separate issue, most likely caused by air in the system.