94 honda accord temperature gauge red lines immediately and when engine is off

I have a 94 honda accord. Recently the temperature gauge will stay in the top red line position. It’s not a slow climb, just immediately when you start the car it will max out, even if the car has been sitting overnight. Coolant is topped off, engine has oil.

since the car doesn’t seem to be overheating we have driven it with the gauge like this. Sometimes while driving the gauge will suddenly fall back to the midpoint reading.

I purchased a new coolant temperature sensor. unplugged the old one (but left it screwed into the coolant loop) and left the wire disconnected then turned the ignition to on (engine still off) temp gauge read maximum. then I plugged the new sensor in and just left it hanging from the wire (so it was connected but not installed into the coolant line) and then turned the ignition to on, and the temperature gauge read maximum.

So do I need a new gauge assembly? Maybe I should also note that my odometer comes and goes as it pleases

read that there are two sensors, one in front of the distributor (near where the coolant line goes to the top of the radiator) and one to the rear of the distributor (near coolant line that goes to the bottom of the radiator) I have tried plugging the brand near coolant sensor into both but the gauge always reads high.

The reason why there’s two sensors is that one is the Coolant Temp Sensor (CTS) which supplies temperature information to the engine computer. The other one will be the sender for the temp gauge, which is the one you need to replace. The two are almost certainly not interchangeable, so you you’ll need to go to the parts store and ask for a “coolant temperature sender” and make sure you make it clear it’s the one for the gauge.

Given the weirdness happening with the odometer I suppose it could be the gauge, too. A way to test it would be to see what the gauge does with the sender wire completely disconnected and then with it grounded to the engine block. It should read completely cold one way and completely hot the other way and if it doesn’t that could indicate a bad gauge or a loose instrument cluster.