A B737 LEVELS OFF AT 13000 FT INSTEAD OF 12000 FT AS SET INTO THE ALT SELECT WINDOW DUE TO THE FO HAVING THE WRONG ALTIMETER SETTING.

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ON DSCNT INTO DTW ON THE POLAR ONE ARR WE WERE CLRED TO CROSS POLAR X-N AT 12000 FT. IT WAS THE FO'S LEG USING THE B AUTOPLT. AS WE DSNDED THROUGH 14000 FT THE ALT ALERTER WENT OFF. I NOTICED IT WAS TOO HIGH. AS I BEGAN TO SEE WHAT WAS WRONG; THE ACFT LEVELED OFF AT 13000 FT. I COULDN'T FIGURE OUT WHAT WAS GOING ON. WAS THERE A PROB WITH THE ALT ALERTER? WHAT WAS HAPPENING? I TOLD THE FO WE WERE TOO HIGH; THAT WE WERE AT 13000 FT NOT 12000 FT. HE SAID WE ARE AT 12000 FT. IT WAS AT THAT TIME THAT I REALIZED THAT HIS ALTIMETER WAS SET AT 29.42 HG INSTEAD OF 30.42 HG; PUTTING US 1000 FT HIGH. I POINTED IT OUT TO HIM. HE RESET TO 30.42 HG; AND BEGAN A QUICK DSCNT TO 12000 FT. IN THE CONFUSION; I AM NOT SURE IF WE WERE ABOVE 12000 FT AT POLAR; AS WE HAD GOTTEN DOWN EARLY; BUT FIGURE WE MAY HAVE BEEN 300-500 FT HIGH. THERE WERE NO COMMENTS FROM ATC. I BELIEVE THAT WHEN WE DID THE DSCNT CHKLIST; I DID NOT CATCH THE MISSED SET ALTIMETER EITHER VISUALLY OR HEARING IT READ. IT SEEMS I HAVE NOTICED QUITE A FEW FO'S ONLY SAY FL180 DSNDING THROUGH FL180 AND DO NOT REPEAT THE ALTIMETER SETTING AS OUR PROCS CALL FOR. THUS; WHEN THE DSCNT CHKLIST IS READ; THE ONLY RESPONSE YOU HEAR IS 'SET AND XCHKED.' I THINK IT WOULD BE A GOOD PROC TO ADD THIS TO THE ALTIMETER'S AND BUG'S CHKLIST INQUIRY TO BACK UP THE FL180 CALLOUT.