Hi Shain, The thermistor in the Idle Speed circuit activates a circuit in the Idle Speed ECU, when the coolant is below 15 degrees C, which tells the ECU to open the idle speed regulator a little more - to allow the engine to idle faster when it is cold, or warming up. As Far as what could happen if it were to fail: I guess (and this is just a guess) it could fail one of two ways: It could remain in one state, after the engine is warmed, telling the ECU to keep the regulator open more and the idle high. Or it could remain in the other state, never telling the ECU to open the regulator more - not letting the engine idle high enough during warm up. I've haven't encountered either problem personally, nor am I sure the thermistor would fail in either of these states - but it seems reasonable, looking at the schematic. Eric VIN 05557 ericp@xxxx --- In dmcnews@xxxx, "dsscid" <shain@xxxx> wrote: > Hello, > > Can someone please tell me what the idle speed thermistor does. I > know it has some part in the idle speed control. Could you also list > what would happen providing the sensor failed. Thanks. > > - Shain