See the CFS Intoxilyzer 8000C Training Aid 2013 p. 136 of 238. "The instrument has detected an unacceptably high alcohol concentration in the ambient air drawn in during the air blank. The ambient fail message is communicated when the difference between the vapours in the sample chamber at cessation of the air blank (i.e. when the pump stops) is more than 10 mg/100 mL higher than the baseline measurement obtained before the pump was activated." With respect, I suggest ambient fail can occur as a result of radio frequency interference AND as a result of the difference between the vapours in the sample chamber being LESS than the baseline floating zero measurement.