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I had my 2000 Mazda Protege transmission rebuilt by what I thought was a good shop. At first everything was fine. I've since found out my transmission model is a CD4E I believe, Frt Wl Dr. About the 4-5 months after the repair the "Check Engine Light" Came on. I had it checked out and was told it was an oxygen sensor code. I didnot replace the sensor. About 8-9 months after the repair I was in for an oil change at a lube place and they told me my transmission fluid was black and if I didn't have it flushed It would dammage the transmission. Scared, I would dammage the transmission, I had them flush the transmission. Shortly after that flush the transmission began to surge and was very hard to drive. I went to a different transmission shop and they told me my transmission was no good. Without the emotional details, I believed them and they again rebuilt my transmission. My question is, could either of the items I mentioned caused the transmission to fail again? The CK engine light or the lube store flush of the transmission?  I need knowledgeable answers please. I'm at my wits end. Thank you kindly.  

the oxygen sensor, no. the flush, yes. unfortunately... you had black fluid prior to the flush which means they probably did not break it, it was already failing. they should have advised you against the flush.  
ASE/ATRA MASTERS REC/EXP, OEM, ATC AUTO DEGREE

a little piece of friendly advice... take it to a dealer for a diagnostic for documentation purposes.  
ASE/ATRA MASTERS REC/EXP, OEM, ATC AUTO DEGREE