With the engine cool, verify the radiator coolant level, check for leaks, and bleed air from the system. If the gauge still rises at idle, test the cooling fans and their low- and high-speed relays, then replace the thermostat and rebleed the system. If the problem continues, the radiator may be failing and should be inspected.
The owner of a 1997 Honda Accord reports that the temperature gauge rises after idling more than about three minutes and drops once the car is moving. Experts recommend checking the actual radiator coolant level rather than relying only on the reservoir, looking for leaks, and bleeding air from the hot system. They also suggest checking cooling-fan operation and relays, replacing the thermostat if the issue continues, and considering a failing radiator as a common possibility. One reply disagrees about the thermostat location, saying it is at the lower hose rather than the upper hose.
