C210 LANDED GEAR UP; MISTAKING GEAR WARNING HORN FOR ALARM INDICATING EXCESSIVE CARBON MONOXIDE.

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WE WERE FLYING A VFR FLT FROM BJC TO CYS WITH THE PLAN TO PRACTICE A DIVERSION TO ZZZ BY PILOTAGE AND DEAD RECKONING. WE DECIDED TO PRACTICE SOME SHORT AND SOFT FIELD TKOF'S AND LNDGS AT ZZZ. ON OUR DSCNT INTO ZZZ AND WHILE IN THE TFC PATTERN WE NOTED THE CARBON MONOXIDE MONITOR ALARM WENT OFF. THIS WAS AN INTERMITTENT PROB AND HAS ON OCCASION; THOUGH NOT OFTEN; BEEN NOTED BEFORE. (I HAD ADDRESSED THIS ISSUE WITH OUR A&P IN THE PAST WHO THOUGHT IT MAY BE DUE TO TURBULENT EXHAUST ENTERING THE GEAR WELLS AND CABIN.) WHILE WE WERE PRACTICING IN THE PATTERN (L CLOSED TFC ON RWY 16) I NOTED THE ALARM APPEARED TO BE SOUNDING AGAIN ON FINAL APCH. THIS BECAME A DISTR FOR US; AND WE BECAME FIXATED ON THE CO MONITOR AS THE PROB WHEN IN FACT THE GEAR HAD NOT BEEN LOWERED AND THE SOUND WE WERE HEARING WAS; IN FACT; THE GEAR WARNING HORN. MISTAKING THE GEAR WARNING HORN FOR THE CO MONITOR ALARM; WE BOTH FAILED TO DO THE FINAL CHKLIST AND VERIFY LNDG GEAR DOWN (GREEN LIGHT). I KNOW THAT HAD THE CO MONITOR ALARM NOT BEEN IN THE ACFT; THE ONLY SOUND I WOULD HAVE IDENTED WAS THE GEARING WARNING HORN; BUT OBVIOUSLY THIS WAS NOT THE CASE. I HAD BECOME FIXATED ON THE CO ALARM SOUNDING TO THE EXCLUSION OF BASIC RESPONSIBILITIES INVOLVED WITH LNDG. WE ARE USUALLY VIGILANT ABOUT CHKING FOR A 'TIRE' AND CHKING THE MIRROR FOR THE NOSEWHEEL AND VERIFYING THE GEAR INDICATOR LIGHT 'ON;' ESPECIALLY SINCE WE HAVE HAD A PROB THIS PAST YR WITH THE GEAR EITHER NOT EXTENDING OR THE GEAR UP INDICATOR LIGHT NOT LIGHTING UP AFTER RETRACTION. WE HAVE HAD THE PLANE WORKED ON FOR THIS PROB AND OUR A&P CAN VERIFY THIS; BUT THIS IS NOT AN EXCUSE FOR OUR MISTAKE. IN ADDITION; TO VERIFYING GEAR DOWN WITH THE CHKLIST AND GUMPS CHK IN THE PATTERN AS WELL AS A FINAL CHK ON FINAL APCH; THE VERIFICATION MUST BE VERBALLY ACKNOWLEDGED BY BOTH PLTS; JUST AS WHEN POSITIVE XFER OF CTL OF ACFT IS PERFORMED. IF ALONE; IT WOULD BE BEST TO VERBALLY ACKNOWLEDGE ALOUD TO ONESELF THAT THE CHKLIST AND GUMPS HAVE BEEN COMPLETED. IN ADDITION; IF ANY WARNING HORN IS HEARD AGAIN; IT WILL FROM THIS EVENT; BE ACKNOWLEDGED THAT THE SOUND MAY IN FACT BE FROM THE GEAR WARNING HORN.