DISTR FLC OF BE02 ERRONEOUSLY DSNDED BELOW ASSIGNED ALT ON ARR TO COS; CO.

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I WAS CAPT ENRTE FROM ABQ TO COS. FROM OUR ENRTE ALT WE HAD BEEN CLRED TO DSND TO 17000 FT THEN PLT'S DISCRETION TO 13000 FT TO CROSS 35 MI S OF BLACK FOREST VOR AT 13000 FT. OUR ACFT HAD BEEN EXPERIENCING MODE C PROBS WITH OUR XPONDER AND WAS SENDING ERRONEOUS INFO TO ZDV. CTR WAS AWARE OF THE SIT. AT THE TIME OF OUR CLRNC TO DSND; I PRESSED THE TEST BUTTON ON OUR XPONDER AND IT WAS ENCODING APPROX 10000 FT ERRONEOUSLY. I MISTAKENLY PUT 10000 FT IN OUR ALT SELECTOR. WHEN CTR HANDED US OFF TO COS APCH; I RPTED OUR ALT AND WAS ASKED TO REPEAT IT. I REPEATED OUR ALT WITHOUT THINKING ABOUT IT AND ON FINAL TO RWY 35R IN COS I REALIZED WHAT I HAD DONE AND WHY CTR HAD ASKED US TO REPEAT OUR ALT (WHICH HAD BEEN 10000 FT INSTEAD OF OUR 13000 FT CLRNC ALT). ONCE ON THE GND IN COS; I CALLED ZDV TO TRY TO CLARIFY THE SIT. I TOLD THEM EXACTLY WHAT I HAD DONE AND THAT I HAD BEEN DISTR BY THE MODE C. THEY SAID THAT THE TAPES HAD BEEN PULLED AND AN INVESTIGATION WOULD FOLLOW. MY DISTR WITH THE XPONDER RESULTED IN MY PLACING THE IMPROPER ALT IN THE ALT SELECTOR.