On a Mazda 6 the engine cooling fan now runs all the time what is wrong?

Answer

most likely it is the engine temp. sensor that tells the fan the motor is hot if it is going bad then it can make the fan run non stop

I have the same problem in mazda6 2004 2.3l engine. If you have $100 you go to the dealer and get the update for compute unit, it is a bug in it. If you do not have money turn of the heater fan than engine is cold, as soon as engine warms up the fan will turn off.