I have a 2004 Honda Civic ex 1.7 ltr. I’ll drive it on the highway and temperature gauge holds steady but as soon as I hit residential streets it starts to climb. Thought it was the fan motor but once I turn the ac on both kick on. It did this the other day I took it to a local mechanic and he got a reading with a temperature gun right by the thermostat and it read 240 while the gauge was in just under the red. I then turn the ac on in the car both fans kicked on however the air that they were blowing was a lot cooler than t should have been. The radiator is a year old and so is the thermostat. Could it be a bad temperature coolant sensor?

  • Welcome to Motor Vehicle Maintenance & Repair! – Cullub Aug 1 '19 at 19:33
  • What fans are you talking about? Please edit your post and make it clear. – mike65535 Aug 2 '19 at 16:23
  • I’m talking about the engine cooling fans. The radiator fan won’t turn on all the time when it starts to overheat. Once I turn the AC on both immediately turn on but don’t always cool the engine down according to the gauge on the dash. – Matt515 Aug 2 '19 at 20:03

You definitely have a engine cooling fan related issue. The fact that things cool at highway speed, but not at stop and go traffic is a dead give away. In your car the engine cooling fans (electric motors) are controlled by an external module. And that module takes as its input the current coolant temperature.

When you use the air conditioning (A/C), there is an override to the engine cooling fan controller. Generally whenever the A/C is engaged the engine cooling fans are forced to high speed, regardless of other settings.

Sure does sound like a temperature sensor input is has gone awry. Note there may be multiple sensors on your vehicle, not sure where they are all located. The sensor used by the instrument panel temp gauge may not be the same sensor as is used by the cooling fan. Its also possible that the cooling fan module internals has troubles, but generally if that happens the A/C override wouldn't work. Its possible the trouble is there, but I'd check all the temp sensors first. Are you handy testing electrical components? Do you have access to a repair manual and a volt ohm tester? You need the repair manual to identify the location of each sensor and the expected electrical resistance from that sensor.

Test the output from each temp sensor. If there is a fault, it will be easy to figure out on the volt ohm meter. Replace the sensor and move on. Good luck with the repair.

  • There are two temperature sensors that I’m aware of on the vehicle. Correct me if I’m wrong but I think the gauge reads off the sensor directly on top of the thermostat housing. I gathered this from when I tried to replace said sensor and a certain auto store gave me the wrong sensor. When I put it in and turned on the engine the next morning so it was completely cold the gauge immediately shot up ubove red. When the mechanic unplugged the other one the fan was not working before he did it. Once he plunged it back in it kicked on. Unfortunately I don’t have access to a tester – Matt515 Aug 2 '19 at 19:59
  • Perhaps you missed some of my comments? Lets try this again. Do you have access to a repair manual and a volt ohm tester? You need the repair manual to identify the location of each sensor and the expected electrical resistance from that sensor. Volt Ohm multimeter is free. . Note you could have as many as four temp sensors in your vehicle. (two in coolant, one in the air intake, ambient air, ...) . YOU NEED A REPAIR MANUAL. Chilton / Haynes are great sources... not much $$. Multimeters are cheap. Get one. Good luck with your repair. – zipzit Aug 2 '19 at 20:21

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