You have FOUR (4) Oxygen Sensors (O2 Sensors), and NO CO2 sensor. There are MANY different O2 Sensor codes that are possible, and MOST of them don't mean you need to change the sensor, so you/your mechanic need to understand what the code is and what it means (what conditions cause that code to be saved in DME memory), and/or how to test the wiring or system. It could be something as simple as a blown O2 Sensor HEATER fuse. Have you had the breather heater recall performed by the dealer yet? That is on the same fuse circuit as the four O2 Sensor heaters.Hi, so BMW wants $1300 to replace or diagnose the OBD CO2 Sensor replacement. However, my mechanic (cheap) replaces it now twice now with a BMW sensor. The check engine light is still coming on and reading CO2 Sensor. WHY????