Suppose to be self purging, have to keep adding coolant after the engine idles for awhile, then it will be low about three days latter.
What can I say if your relays are good but no fan, and ETC can be way out of tolerance, or code stored in flashram that can be corrupted. High resistance in the terminals or that thermistor itself makes the computer thing the engine is much colder than it actually is.
Typical resistance at around 200*F is around 200 ohms, if you are measuring say 275 ohms, your fans will not kick on at all. And will only get an error code if this sensor is completely opened or shorted. Be darn if I can find the spec in the shop manual, aholes say just to replace it with a known good one. How do you know its good or bad if they can't even give this simple specification to test it?
Not this familiar with the 1.8, the 1.4 has a DIC digital readout for the engine temperature, this can be compared with the actual temperature at the base of the thermostat. Should be about the same, nice to have accurate temperature measuring equipment.
Are you sure that plastic to your reservoir is the only problem? Have plastic in other spots as well, and an O'ring on the reservoir cap. How many miles on this thing?