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I just bought an 09 Honda Civic with fairly low mileage. Within 3 days it started to overheat so, my boyfriend replaced the thermostat and all of the cooling lines as they were dry rotted (like I said fairly low milage on this car). The car gave me no issues for a week after all of this was done. Well, as of last night it started overheating again. Any ideas as to why would be greatly appreciated.

  • Do you have any history of the car services? Specifically, is the water pump ever replaced? Are there any coolant leaks visible? What is the condition of the radiator cap? Is anything blocking the air to flow into the radiator? – Alimba Sep 13 at 10:07
  • I have no visible leaks, that was the first thing I checked last night when it started to overheat. The radiator cap appears to be in good condition and the service records I do have on hand don't mention anything about the water pump ever being replaced. The radiator has clear airflow. My drive to work this morning the temperature gauge fluctuated between normal operating temperature and low, as it was cool with low humidity. It is now very warm and humid out, so I am dreading the drive home. I have a feeling there is going to be an overheating issue again – Ashley Sep 13 at 18:38
  • I have got myself home before now by putting the heater on hot and the blower full - with the windows open... can just make enough difference between a blown head gasket and not... – Solar Mike Sep 13 at 21:07
  • I see, how you tell that the car is overheating? Usually, it has power loss and the coolant may start to overflow, the radiator fan will work overtime. From your last comment I started to suspect that the gauge is not to be trusted. – Alimba Sep 14 at 8:05
  • @SolarMike Head gasket is a possibility, but the car has low mileage and it sounds a bit unlikely. – Alimba Sep 14 at 8:06
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According to the comments above it seems that the water temperature system is the one to blame for this issue.

You could use some electrical contact cleaner on the terminals of the water temp sensor, because the problem might be just moisture getting into the connection. If that fails try to test the sensor with the following...

In theory you could test the sensor with a voltmeter but you have to find the specs for it. Usually they are 2 tests you do, hot and cold, in a form of e.g. : 25° = 0.5 volts and 80° = 1.5 volts

If you end up replacing the sensor, be aware that replacing the temp sender unit it will require you to drain the coolant and either put it back in or replace it.

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