Codes 0171/0174 tell you both sides of the engine, all 6 cylinders, were running too lean, too much "air" in the exhaust.
This could have been a one time event or a long term issue, I would reset computer and see how long before CEL comes on again.
Unhook negative battery terminal for 5 minutes, this will cause computer to reset, codes will still be in the "history" file, but CEL will be off unless codes reoccur.
If CEL comes back on within 5 minutes(cold engine) then it's a computer issue.
If CEL comes back on just after warm up then it is most likely an air leak, vacuum leak is the usual suspect, cracked or loose vacuum hose, dirty or bad PCV valve.
If it takes longer for CEL to come on then could be a fuel pressure issue, '98 should have 65psi of pressure, you can rent fuel pressure testers.
Temp gauge could be related or not.
Lean mix will cause engine to heat up, but it would go above normal so not sure if we are on the same page.
"Normal" means where needle usually is after warming up, right?
If so then needle would go above this with lean mix if you were accelerating then drop down to "normal" when cruising.
Dropping below "normal" means thermostat has opened allowing warm engine coolant to flow thru radiator, this cooler rad coolant then flows into the engine causing gauge to go down a bit until system temp is equalized.
On cold days this can be more noticeable.
But continued drop below normal could mean thermostat is starting to fail, it is opening too much, in cold weather this can produce the up and down needle, but never above "normal", unless it sticks closed, then you get overheating.