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My Canadian purchased 2004 Honda Civic Si has developed a cold heart. For the 1st time ever (weather is no excuse) the temperature guage will barely move above the C. Warm air comes out of the vents, but not hot like before. Is this the thermostat stuck in the full open position? Is there a sensor in the engine which may be failing. If I drive it long enough, and I mean 30-40 minutes, it sometimes makes a run for the normal zone in the guage, but quickly drops back to cold when slowing down or stopped at lights. Where is the thermostat located on this model. I'm sure I can change it myself, but any tips are welcome.

 
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Nobody?

 
Rep Power: 313

Start by changing your thermostat (its by the intake mani) and doing a flush on your coolant system. I know you have been writing about thinking your head gasket is blown this may cause the effect.

 
Rep Power: 811

has to be thermostat if the car eventually warms up after alot of driving. even with a working thermostat it can be hard for the motor to stay at operating temp if its really cold outside. make sure you only use an oem honda thermostat.