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I am at my wit’s end regarding an overheating issue with my son’s 2010 Civic DX VP.
Initially, it was losing water and overheating. We could not determine where it was losing it from. There were no puddles anywhere or wet areas noticeable. A cracked head or blown head gasket came to mind, but there is no water in the oil, and the engine runs great. Realizing that it may be a leak that only occurs when the engine is warm, we started looking elsewhere and found a faulty lower seam on the radiator. At the same time we found the radiator fan to be broken. We replaced both the fan and the radiator, as well as the hoses and cap. The issue seemed to be fixed, although it may have just been short drives masking the problem. After about a week, the overheating returned. We then replaced the thermostat. Still overheating. The radiator fan would only come on with the A/C fan when the engine was quite hot. My son took it to AAA and they said they thought it was a faulty ECM. Before doing that I tested and verified that all the relays were good (not just the fan related ones), as well as the fan control switch in the fuse box and all the fuses. Then I replaced the ECM and had it reprogrammed. The car continued to overheat. At this point about the only thing left that I could think of was the #2 ECT sensor, so I replaced that, too. I took the car on an extended drive (two trips of ~90 minutes each) with my Innova unit hooked up monitoring live ECT data. The ECT ran at about 217-232 most of the time, but would run up to 244 on occasion and once hit 250, at which time I had to take evasive action.
The fan comes on religiously at high temperature with the A/C fan, but never by itself.

I don’t know where to go from here. I know the fan supposedly has a low and high speed and evidently is not running in low speed. Is this a fan issue or a circuit issue? Any other ideas or thoughts??

Thanks in advance.
 

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driver side fan is the main cooling fan, passenger is the AC fan.

I would check fuses and relays.
I would also try running the car WITHOUT the thermostat.

I have seen DOZENS of thermostats junk out of the brand new box. All it takes is a good drop during shipping to the store and a whole case of them can be junk.


Running without the thermostat should prove that it is not any sort of mechanical problem like a water pump taking a crap, and should also help prove that you simply are not being blocked by a stuck thermostat.

The sensor you are reading with your scan tool is NOT read by the fan switch to turn the fans on.

The ecu is NOT responsible for all fan function. That is why the fan works with the AC.

I am NOT ruling out a bad power wire, but I merely suggest you remove the thermostat and investigate further!
 

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You can also try using a multimeter, and even while car is cold after sitting, unplug both coolant sensors, (radiator mounted, and engine block mounted near throttlebody) and emasure resistance.

I do not have a chart to post for you, but they should read the same resistance if they are reading the same temp.

They are thermoresistors, meaning testing them with them unplugged is fine so you can touch the sensor pins.

Dont turn key over at all while testing. One or both unplugged will cause some check engine lights and codes to be set.
 
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