If it ain’t broke don’t fix it

If it ain’t broke don’t fix it

and IIABDFI
sent. & comp. abb. Leave well enough alone.; Don’t tamper with something that is all right as it is. It’s fine. Leave it alone. If it ain’t broke, don’t fix it!
See also: broke, fix, if
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.
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