Converter temperatures may drop so low that the converter literally stops functioning. If a vehicle has high mileage and a marginal converter it may take an extended preconditioning period to get the converter to light-off and become operational. Vapor canisters become charged with fuel vapors. Canister charging with fuel vapors is a normal function, especially during hot ambient conditions. Many computer control strategies are employed by manufacturers to purge the canister.
Crankcase dilution can be another source of fuel vapors during transient testing.
Extended oil change intervals, overly rich mixtures or a ruptured fuel diaphragm can contribute to crankcase dilution. This is especially true during the cold weather season, when vehicles may have extended warm-up intervals. If the engine is not fully warmed-up before entering the emissions testing sequence, high CO and HC emissions can result. However, this does not assure adequate converter or engine temperatures. By establishing a vehicle precondition protocol or procedure.