If I remember correctly the ECT sensor does affect your idle because it is used as part of a feedback loop for you IAC valve. So if your ECT is a constant high (which would indicate low temp) or a constant low (indicating high temp) or fluctuating then your IAC wouldn't know what to do. There are some tests you can do with a digital multimeter but you would have to look at a chiltons since I can't remember what they are right off hand. Hope this helps.