I have had my 2008 Chevrolet Spark for 11 years and it has been super reliable but on Wednesday it overheated on a hot day in a motorway traffic jam. I removed the thermostat, put in some water and I did complete my journey. I didn't think it was the thermostat but it was an easy thing to try with the few tools I had in the car. I suspected the fan but today I tested that by applying 12v to the motor and it works. What I did find was a white ceramic component near the radiator which looked cracked. This turns out to be the cooling fan resistor, a part I had never heard of. I tried to remove it but the connection screws had corroded and seized. A lump of the ceramic fell off when trying to undo the screws. What does this part do, is it a 2 speed fan and this gives the slow speed? If this part has failed would it cause the overheating? I intend to rig up a manual switch for the fan and possibly some kind of generic temperature gauge. The Spark does not have a temperature gauge and I now know you get about 5 seconds between the temperature warning light coming on and steam pouring out!