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no reason to change coolant, only topup and bleed after thermostat change with your regular 50/50 mix coolant.
have not you noticed an erratic temp gauge? it should go vertical and stay there within about 5 minutes after starting if thermostat is good.
as far o2 sensors, have they exceeded or near their recommended mileage limits? as i understand it, the '98 has 4 so its gets expensive, unless only 1 has crapped at an early life mileage. so a diagnostic test may be involved to find the bad one.
in my area you only need to have no obc2 check engine lights on to pass emmissions. no sniffer is needed in your tailpipe. so, if your light goes away with new thermostat- if indeed it is needed- than you should pass your next inspection. if this happens, you have no need to then confront the o2 sensor issue.
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