Heating maintenance. Checked most of tasks upon arrival. Customer stated had a water leak and wanted to make sure everything looked okay. Period went ahead and started maintenance with combustion reading, and immediately had readings skyrocket until my meter pinged out when ahead and wanted to double check this stepped outside had zero out before I turned off meter then proceeded to turn back on and re-calibrate with fresh air from outside, came back inside, took test again, and had same results. At this point system is unsafe to run and went ahead and red tag system. Recommended replacement of system and will have a design specialist out here to give them those options. Make sure system could not run and heating mode.