I have a 93 Honda Accord, the temperature gauge goes immediately from C to H when I just turn the key, not even start the engine. I suspect it's the thermostat or the temperature sensor, so I replaced both, but the gauge still doing the same thing . Appreciated if anyone have any suggestions for the next thing I should check? Thanks.

 

which temperature sensor did you replace? How many wire(s) were going to that sensor?

 

On the end of the head, kinda below the distributor, look for a small-ish sensor with only one wire. Unplug that & see what the gauge does.

Any sensor with a 2-wire plug doesn't have anything to do with the dashboard temperature gauge. Your car has another 3 devices that you might think are temperature senders...