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.
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.