GA SMT ALT DEVIATION UNDERSHOT CROSSING RESTRICTION.

Narrative

ENRTE FROM SCK TO APA AT FL250 HEATER QUIT WITH 1 1/2 HRS TO GO. BECAME COLDER BUT NOWHERE NEAR DEBILITATING. NEVERTHELESS; DEFINITELY A PREOCCUPATION. THOUGHT ABOUT ADVISING CTLRS AT SLC OR DEN CENTER ABOUT SITUATION BUT DIDN'T THINK IT WOULD CAUSE A PROB. CLRED TO DESCENDED AND THEN TO BYSON INTXN; I THOUGHT TO CROSS BYSON AT OR ABOVE 16000'. I'M FAIRLY CERTAIN THAT I READ IT BACK THAT WAY. AFTER BYSON; I WAS HANDED OFF TO THE NEXT CTLR WHO SAID THAT THE PREVIOUS ONE WANTED TO KNOW WHY I HAD NOT CROSSED AT 16000'. I TOLD HIM THAT I THOUGHT I WAS TO CROSS AT OR ABOVE. THEN; APCHING THE ARPT; I NOTICED THAT THE LIGHTS ON MY AVIONICS WERE OFF. BECAUSE OF THE COLD I HAD PUT ON A SPORT JACKET AND THEN A MASSIVE; BULKY OVERCOAT. A BUCKLE ON THE COAT HAD SNAGGED THE AVIONICS MASTER SWITCH AND TURNED THE AVIONICS OFF. I TURNED THEM BACK ON AND ASKED APCH IF HE HAD BEEN TRYING TO GET ME AND HE REPLIED AFFIRMATIVELY. THINGS WOULD HAVE BEEN A LOT EASIER IF I HAD NOTIFIED THE CTLR WHEN THE HEATER FAILED. IT WOULD HAVE NOT ONLY ALERTED ATC; BUT I THINK IT WOULD HAVE RAISED MY OWN AWARENESS THAT THIS WAS; INDEED; AN UNUSUAL SITUATION THAT REQUIRED EXTRA VIGILANCE.