There are technically 3 things watching coolant temp. fan switch, gauge sensor, ecu sensor
Find the downloadable technical manual for your car online, and look for the temp sensor chart that references resistance value versus temp.
Then unplug the gauge-reading sensor on the side of the cylinder head or water neck, depending year, and measure the resistance across the terminals. match it to a value that best matches your ambient temperature, its okay to be off 10-20 degrees.
normal operating temp hovers usually between 185 and 200.
Id do this cold with the key turned on to light gauges up, that way you can easily get a reading of ambient or cold, and get resistance value.
Lets say its 15 ohms off from your ambient. Now you have your delta difference, and can use that as a tool to see if its causing the gauge to read high, but the engine is actually doing just fine.
I put in a honda genuine tstat. I dug out my meter and the wires were sliced have to pick up a new one tomorrow.
1997 honda civic D16y7 shortblock Y8 head with a y8 harness fan wouldnt kick on before so i put in my new fan switch sensor. Thought it was my dual core alunimum radiator so i switched it with a new OEM radiator at the same time, not leaking coolant and no oil coolant mixture so its leading me to believe my HG is fine.
I have had those single wire temp sensor act funky on me before.
Also i always have a servicw manuel PDF handy as well. Was seeing if there was an easy diagnosis with this like say switch parts. I have tons of new and used sensores blocks parts etc.