One problem related to o2 sensor has been reported for the 2004 Mazda RX8. The most recently reported issues are listed below.
2004 Mazda RX 8 has many design defects, battery was replaced, spark plugs and oil filter was replaced 3 months after purchase. The started was replaced. The vehicle was stalling in traffic and was taking multiple times to try to restart the vehicle. After numerous times of the vehicle being towed, the dealership decided to replace the entire engine. After the replacement of the engine, the engine light illuminated and the vehicle was still not starting smoothly. The dash lights were blinking on and off and the heat was not working. The catalytic converter, 02 sensor, and a/c amplifier. The heater control switch was replaced. The sun visor was replaced.