There are a few problems with getting a smog check if your vehicle has a blown head gasket. First off, the smog inspection consists of a visual inspection which checks for white and black smoke being emitted from either the engine or the tailpipe. If the blown head gasket is causing steam (white smoke due to internal coolant leakage) or black smoke (rich air/fuel or internal oil leakage) to emit from either the tailpipe to engine compartment, your vehicle with fail the visual portion of the smog inspection, and subsequently, the engine smog check.
If there is no visible smoke or steam from the vehicle, and the vehicle is model year 1999 or older, it will be driven on a dynomometer during the smog test and an emission sample be collected from the tailpipe; more than likely the head gasket problem will cause high NOx emissions which will cause a smog check failure. We also don't recommend getting a smog check while your vehicle has a leaking head gasket. NOx is created when engine combustion temps rise above 2500f. A leaking head gasket will almost always cause increased engine temperatures. If any water (or oil) is leaking into the engine combustion chambers as well the coolant system your vehicle may also produce high HC and CO.
If your vehicle is model year 2000 and newer, it will not be driven on a dynomometer and its emissions (from tailpipe) will not be collected or inspected (measured). In this particular case, a vehicle may pass the smog check if its check engine light is not on (illuminated) and no trouble code(s) stored in the ECU (engine control unit), AND the vehicle not display blown head gasket symptoms. The vehicle must be performing at or above California Air Resources Board set emission standards. Should the smog technician notice a blown headgasket fault, they will be obligated to enter this information into the smog check machine; and fail the vehicle for "defective mechanical fault".
