One problem related to engine exhaust system has been reported for the 2004 Mazda MAZDA6. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Mazda MAZDA6 based on all problems reported for the 2004 MAZDA6.
Pre catalytic converter failed, destroying my v6 engine. I thought it was a joke, but I heard several cases with other people with the same vehicle; 2004 Mazda 6s. $3000 for 4 new catalytic converters and $4000 for a new v6 engine=$7000. . . . I don't think so.