No heat call. Numerous replacement limit control replaced. Found a brand new limit in place. After starting the unit if was off on limit again. Bypassed limit to perform a combustion analysis. The CO PPM was well over 2000 before I shut the analyzer off to prevent damage. Burn marks on exterior grill and loose gas valve connections inside the unit. I disconnected gas from the utility closet and shut the breaker off in the apartment. Unit will need to be replaced. Pictures and a brief description of diagnostic has been emailed to Jane.