Helga, 135k,mostly stock...my story today...car running fine until today...heavy, unusual for Central NY this time of year downpour overnight...lightning, hail, the whole weather spectrum...got in car this AM, crank but no start...figured moisture in distributor cap, got cap off, seemed dry enough, sprayed/wiped down with WD40, re-install...crank, crank, crank, finally starts...let it run for a few minutes before taking off to run several errands around town...noted temp gauge holding at a line or line and a half cooler than normal, and at each stop, electric fan kicks on, which it doesn't normally do unless really warm/after hard driving...on way home, check engine light comes on...yay!...again, fan comes on with coolant shutoff...connect code reader, comes up PO 117...issue with coolant circuit...clear code...start engine, check engine light comes on AGAIN, hook up reader one more time, now PO116, slightly different cooling system issue...clear code, start engine, now temperature gauge needle doesn't move at all, and car started a bit harder...did I disturb wiring in dizzy cap r'n'r?...two vacuum lines attach to airbox, between box and battery, one black and one white...if I reversed them, would that cause this issue?...or did engine temp sensor just suddenly die?...

thanks to all the learned in advance...

Tom

Your engine coolant temp sensor died. You might want to check the wiring at the ECT sensor, it may be broken or loose.

Just remember to drain a quart of antifreeze from the radiator before pulling the sensor out. No need to get antifreeze all over the alternator!

Yes, a bad ECT will cause the engine fan to come on and will also cause hard starts.
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Keeping it running is better than buying new

thanks again as always Klaus...I poked around on the interweb last nite, and all indications were that ECT failed....didn't think it could happen so fast, but who knows...likely original to car, since I've owned it for 90k now...part ordered at FCP Euro....

yet another question...could my possibly mis-connecting the black and white vacuum lines going to air cleaner box have had a hand in the ECT demise? or just coincidence...which IS correct?...black or white vac line on top?

This might be of help, it is for an NA engine:

https://www.matthewsvolvosite.com/forums/viewtopic.php?t=65002
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Keeping it running is better than buying new

Thank you yet again KC..

No to your coincidence question. The 1998 is an NA engine which I forgot where the lines go. One goes to the airbox for the heated air intake. The other goes to the throttle body?? Best if you follow the end of the vacuum lines to see where they come from and use a little logic as to what gets vacuum from these lines.

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Keeping it running is better than buying new