2004 and some early 2005 Mazda 3 were equipped with a plastic pulley on the serpentine belt tensioner. This applies to both 2.0 liter and 2.3 liter engines. In newer models it appears as though the plastic pulley has been replaced with a metal pulley instead.
The reason this has been brought up to my attention is that failure to recognize that the pulley has broken by the driver may cause the engine to over heat which can then result in major engine failure if the driver ignores these signs and continues to drive the vehicle.
Typically what happens when the pulley breaks is that you may hear some noise from the passenger side of the engine compartment. Your engine light will turn on as the alternator is no longer being driven by the belt. Prior to the failure, you may hear a loud squeaking prior to failure of the pulley.
If any of the above was to occur to your car, immediately get off the road, shut the vehicles engine off to prevent damage and have it inspected by a trusted mechanic.
Ideally you should have the tensioner inspected by your mechanic at your next oil change or even proactive and have it replaced along with new belts.
Thanks goes out to Glenn of Cam Tech Automotive of Mississauga, Ontario for the pictures below and the tip. A 2004 Mazda3 came in to Cam Tech Automotives shop earlier this week as a result of the pulley failing. The driver continued to operate the car which led to the engine overheating and damaging the cylinder head past the point of repair. Had the driver been aware of the consequences of ignoring the signs, they could have saved a substantial repair bill.
Note that the replacement parts should now be using a metal pulley and no longer the plastic variant.