How do I diagnose the thermostat?
It's a farkin' TS.
It's a farkin' maint./ item.
Get a farkin' new one. jk or lol or haha
Seriously, heard 4 decades ago if'n ya overheat real bad once then your TS might end up toast. Long story about my wife's Explorer -- yah so won't dare bother with details, but end story that's what I confirmed 17 years ago using it to take a stove to a colleague.
\'96SS SOB: SSRI, Herter Tune, Tri-Y II, , 3000 Edge, F/HO bars, METCO extendeds with CV MMC, Bilsteins, currently Vredestein Ultrac Sessanta (315/35 rear). Finally, wait for it... LT-4 knock module!
\'96FWB BUW-Butt Ugly White: PCMPerformance tune, HDFA w/ swissed filter, 1/2 SLP s.s. catback with H- and Flow Super 50s, oem opti rebuilt 3 times, SS Bars, METCO LCA, Airlift 1000, Monroe SS, PowerTrax No-Slip, CTS wheels with +1 rubber. Finally, yes again... LT-4 knock module!