Join Date: Oct 2011
Location: Magnetic Hill, New-Brunswick
Well I hate like hell to disagree but I'm going to anyway. First thing you should check, and I assume you did, is the coolant level. The thermostat itself has nothing to do with turning on the fan. You said earlier that the fan comes on with A/C so this eliminates fan speed 1,relay and fuse. The two fan speeds are determined by coolant temperature. The PCM commands Low Speed Fans ON under the following conditions:
• Engine coolant temperature (ECT) exceeds approximately 106°C (223°F).
• A/C refrigerant pressure exceeds 1 310 kPa (190 psi).
• After the vehicle is shut off, the ECT at key-off is greater than 140°C (284°F) and system voltage is more than 12 volts. The fans will stay on for approximately 3 minutes.
The PCM commands High Speed Fans ON under the following conditions:
• ECT reaches 110°C (230°F).
• A/C refrigerant pressure exceeds 1 655 kPa (240 psi).
• When certain DTCs set
Now with this said you know your coolant temperature sensor is working because you could read 257°F right. So why didn't the fans come on? Something failed with your ECM because this is the module that turns the fans on. You either have a burnt fuse or a broken wire or bad connection somewhere that prevents the "ECM" from receiving coolant temperature readings.
Myself I would do a fan test with a TechII to verify if these work properly first, if so I would then clean the two ECM connectors and re-install. If this fails to fix the problem I would then attempt to find why the "ECM" is not getting the temperature reading.
Might even be your water pump that has seized and not circulating coolant.
Thermostat don't usually just close shut all of a sudden in fact the big problem with Cobalt thermostat is that they don't close enough as they get older.
OKay I'm done.
Never drive faster than your guardian angel can fly
Last edited by Coby7; 09-12-2012 at 05:12 PM.