Darren Potter had an early cross saved and Ross Wallace had an inswinging right-wing corner and a 20-yard shot saved by Owls keeper Lee Grant.
Potter and Chris Sedgwick had crosses saved before Wednesday made it 1-0 after half an hour. Sean McAllister beat Eddie Nolan and from the right-hand side assisted with a low cross which Francis Jeffers converted from close range into the bottom left.
Darren Carter cut in from the left and beat Richard Wood, but his low right-foot shot was smothered by Grant at the foot of the near post.
Stephen Elliott had a cross saved and then Michael Gray crossed for Marcus Tudgay to have a shot saved.
A stinging shot by Gray was pushed around the far post one-handed by the Preston keeper and just before half time a left-wing cross by Wallace was met by Parkin from eight yards out, but he hit the crossbar.
Tudgay crossed for Gray and his shot was saved and Jermaine Johnson curled a right-foot shot against the far post in the 50th minute.
Gray and Johnson had efforts saved before Billy Jones had a 25-yard dipping shot saved.
Jeffers had an effort saved and then Elliott had a shot saved from a corner.
Substitute Youl Mawene had an effort saved from a Nicholson left-wing corner and then Elliott found Nolan, but his shot was blocked.
Lewis Buxton, Parkin and Johnson had efforts saved, but in the second minute of added time Sedgwick had a first-time shot blocked by Wood and Parkin equalised placing the ball low into the middle of the goal.