I bought 2 last year from Amazon and needed a rebuild more than a tune up. I found one if them today and it has no markings or anything printed on it or on connector. Is there any way I can check if it's the upstream or downstream? I have a multimeter and a 12v battery? Thanks

 

If they’re RX8 specific then the one with the short cable should be secondary / downstream / post cat converter(looks like bottom pic)

 

Ok thanks!

 

As long as the wire colors are the same its the same type of sensor. there are one wire two wire ,four wire and five wire sensors they each do different things. cable and plug difference is just location and vehicle specific applications to get the best performance stick to a bosh or ntk sensor look on ntk website and i think i saw a chart the show what each wire does