[collinc]

Hello , First time question. Just bought my wife a 2003 ML 320. Excellent condition. Just replaced the passenger front door actuator and the alternator. Everything seems fine ,and after 2 days the check engine light comes on and the scanner reads that the front RT. O 2 sensor is not getting a reading until the end of the scan. It flat lines until the end while the other 3 get a significant reading while revving the engine.Should I replace the two front O2 sensors,just the one or all 4. Thanks