Car overheating

I have a 2002 Mazda millenia 2.5

I’m having problems with it overheating in certain circumstances, not all the time but it happens once in a while when is very hot ouside in stop and go traffic. I changed the thermostat, put plenty of 50-50 anti-freeze but that didn’t fix the problem. Yesterday when the temperature needle was almost at the top I popped the hood while the engine still running with the AC turned off, and noticed the the AC fan was on and the other fan wasn’t spinnig eventhough the engine was overheating. Are they suppose to kick in at the same time? Also are they both suppose to kick in when i turn the AC or just one fan.

If I have a fan problem how do i find out exactly what it is. I heard it could be something called a relay (where do I find that), I heard that it could be cooling sensor, or the fan. HOW DO I RULE EACH ONE OF THEM OUT? I don’t want to spend a fortune taking it to the dealer or buying unnesary replacements.

You can use a multimeter to see if the fan is getting power. Hook-up the multimeter, start the engine, and turn on the A/C. Irregardless of the engine temperature (hopefully dead cold), turning on the A/C should power up both fans. If power, but no action, the fan motor is burned out. If no power, then hunt down and check the fan relay. You can use some wire to jump the fan directly to the battery to confirm the fan is OK.

I just removed the fan and hot wired it to the battery and it worked, so it’s not the fan. The fan fuses are good so it’s either the relay or the cooling sensor. Which one is the cooling fan relay, can someone tell me? (show me a diagram or pictures if possible)

Thank you!