Any amount of "visible" smoke from either the engine compartment or the tailpipe will cause a vehicle to fail the smog inspection; even if it passes the emissions portion of the smog check.
Smoke should not be confused with steam. It is normal for a vehicle to emit steam from the tailpipe on colder days. A vehicle should however be prepped before the smog test by engine idling in order to eliminate any steam, as well as produce optimum smog test results (applicable to vehicles undergoing a BAR-97 smog check where the vehicle's exhaust sample is analyzed).
