My 95 mazda millenum 2.5 over heats at idle and driving so I checked the thermostat and It works fine.
The fans seem to work when they want to. the left side never works, and the driverside works sometimes. No blockage I can find can you help.

 

Did this problem happen all of a sudden or gradually over time? The thermostat could not be opening all the way open, first thing. You cannot check a thermostst while watching it run. The water pump impeller can be breaking down not pushing water. There could be clogged runners in the radiator. And the most serious problem, head gasket leaking into coolant passage.

The driver side cooling fan is the primary fan that operates when temp gets hot enough. The other is for the A/C when turned on.

 

check thermostat if its working.. also check the head gasket it might be leaking..

 

since this is a topic that i would like to add onto, i've noticed an increasing problem for which i cannot quite explain why it happens, but here it is...... the radiator coolant, most coolants say they will last up to 150,000 miles, have only been lasting about half of that. As a result, they will then start to gel up and clog your radiator thus causing your vehicle to overheat. I've seen 3 cases of this in the past couple of weeks. hell, i had to pick my uncle up yesterday because his car overheated from that happening haha. his car only had 52,000 miles on that coolant. can anyone explain to me why this happens? havent talked to a mechanic about that yet.

 

Check the radiator first off. Let the engine warm up untill you notice it is getting ready to overheat then shut off. Start putting your hand on whatever part of the radiator that you can touch. It should be too hot too keep your hand on over the entire surface from side to side and top to bottom. Disconnect the fan connector before reaching past fan blade so it does not start up with your hand in there of course. If you find any areas that are cool then the radiator has restrictions and will need repaired/replaced.

Gray mazda6, Never think your coolant will last for 150k miles. It has additives just like motor oil or any other fluid in your car that breakdown over time. Additives that prevent metal from rusting, ones that provide additional lubrication for water pump, and most important the ones that raise the boiling point and lower freezing point. Coolant should be replaced every year if possible and every other if it has too. Remember that unless you get a complete system flush, a drain and refill only replaces less than half of your coolant and sometimes less.
Coolant gel up is most often caused by mixing different styles of coolant (ie mixing standard 'green' with extended life 'orange', or premium 'yellow' and it does not take much either. Say your car has 'orange' Dexcool coolant and you are a quart low when taking in for an oil change. The lube place tops up with green as they cannot see any in your overflow so do not know and probably do not care what the correct coolant should be used. 2-3 weeks later after they have been throughly mixed up and heated and cooled a number of times it starts to gel up. Same receipe for making jello isn't it?