Line your bottom oven rack with aluminum foil.
Lay 1 or two cast iron pans upside down on the top rack, not touching one another.
Close and lock oven and set to self clean for 3 or 4 hours.
After cycle is finished, open oven and allow pans to fully cool. May take over an hour. Once cool, use a wire brush or brillo to remove any rush spots and residue. Was with a bit of dish soap and water, dry fully with paper towels and then fully dry on your stove for a few minutes over low heat.
Once cool enough to handle, re-season and there you are, good as new!
I have done this with success on multiple pans, however do so at your own rish. If a pan has a crack in it it may make it worse. OR a small chance exists that it may crack a pan. Only do this with cast iron pans that are fully cast iron (no wooden or plastic handles).