The smog test does not test the muffler but... we don't recommend you take
your car in for testing if you can tell it's fails; either due to loud noise
coming from it or if it has large holes or cracks in it. At best, it may
cause the smog technician a problem with the emissions tailpipe probe, or
dilute your vehicle exhaust to the point the smog machine will not allow the
smog technician to even begin the smog check.
If the muffler is intact but has holes consider this: Smog machines are
calibrated in California (every three days) and are designed not to allow
the testing of vehicles which emit oxygen levels exceeding the smog machines
preset limits. If the holes on a muffler were large enough to where fresh
air would be vacuumed into a car's exhaust stream, the test results would be
incorrect, and the smog machine may not even allow the smog inspection to
begin or continue. Worst case scenario would be the smog test begins, and is
aborted halfway. You'd be required to pay the smog inspection fee, without
getting smog check results or a smog check certificate. We have to recommend
you get the muffler replaced before the smog test.
Also consider this... If there are no visible holes or cracks on
the muffler, and it is only loud, there is the possibility that there is an
internal blockage due to material break down. The blockage will cause a
restriction in exhaust flow. Restricting the exhaust flow will most
definitely cause your vehicle to operate under stress, causing increased
emissions across the board. We'd recommend, if it's not expensive, go ahead
and get it fixed before the smog test. You'll end up saving time.