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?
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.