While trying to get a downstream 02 sensor, all the parts stores only carry upstream 02 sensors. Then I did some research on this issue, It turns out only one 02 sensor on this 1994 Mercedes E320 wagon. Is that true? There is no downstream sensor beyond the catayltic converter in parts stores. So I assumed that the upstream 02 sensor is the one only 02 sensor and downstream 02 sensor is the one and the same as 02 sensor downstream? I thought most cars have two 02 sensors, downstream and upstream. Apparently, my 1994 E320 wagon has only one 02 sensor overall. Is this true? Feedback, comments, advice is greatly appreciated.