TPI 712A740OIL Combines: 712 Deluxe Combustion Efficiency Analyzer with Optional PC Interface, A740 Infrared Printer, A773 Sulfur Filter, A788 Smoke Test Pump with Filter Paper and A790 Analog Oil Pressure Gauge - The carbon monoxide sensor used in TPI combustion efficiency analyzers feature an on-board filter capable of filtering NO (nitric oxide), NO2 (nitrogen dioxide) and NOX (combination of NO and NO2) to prevent cross sensitivity errors.