Performed diagnostic on 18yr old carrier unit, upon arrival customer stated unit wasn’t heating properly, checked tstat, already in heat mode, upon going in crawl/basement found unit not working, unit was giving code 13 (limit lockout) reset power and unit started up, upon coming my co detector started going off, looked at fle vent and you can see exhaust coming back in crawl space, noticed burners was heavily rusted, upon checking roof cap, roof cap is heavily rusted and cap had falling off, inspected heat exchanger noticed a hairline crack and heavily rusted, shut unit off and due to safety reasons, spoke with customer about issues and opt for install supervisor, scheduled for today, unit not working at time