Car fan won't turn on unless AC is on?

I have a 1997 Honda Accord and lately it has been overheating. The fan in the car will not come on unless I run the AC but yesterday I was driving on a cool day with the AC on and it still started to over heat so I pulled over and let it cool down. After that, the check engine light came on.
Please let me know if you know of anything that might be causing this so I can get it fixed. Thank you.
P. S. I have already tried replacing the thermostat and it didn't fix it.

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