26 including tailpipe probe 34 is operable to sample the gasses exiting the tailpipe and determine levels of various constituents such as hydrocarbon hc oxygen o 2 or carbon dioxide co 2 catalyst activity testing must be done on a hot catalytic converter more particularly the converter must have reached its catalyst 34 light off 34 temperature which is that temperature at which the catalyst becomes sufficiently active to alter the rate of the chemical reaction in the converter the engine 10 is operated for a sufficient period of time to allow converter light off and is turned off following engine shut off the fuel and ignition systems of engine 10 are disabled by disabling these systems the engine can still be cycled or cranked using the starter motor but the engine cylinders will not receive fuel or a combustion initiating spark which would interfere with the test results the engine may at this time be cycled for a short period of time to clear the engine cylinders and the exhaust system 12 upstream of converter 20 of any residual fuel or exhaust gasses following the engine purge the hydrocarbon enrichment apparatus 24 is connected to the engine 10 via propane supply conduit 32 and the exhaust gas analyzer 26 is connected to the exhaust system 12 via probe 34 the attachment location of the propane supply conduit 32 of the enrichment apparatus 24 may be any location which is upstream of and.