I was always under the impression that any broken links are a bad idea for ranking, and visitor experience obviously.
Try
Xenu, it`s free and very good.
I think it`s good advice to run Xenu every 6 to 12 months.
A quick bit of advice :
is there any way of Xenu showing where on the page the broken link is ?
The best way I`ve found :
Get the Xenu report (the browser version).
If you can`t find a link that`s listed in the above check through the Xenu file list on your PC. This lists all the checked links and resources but can be very long, it certainly is for my site ! The broken links are in red and the link text is in the fifth column listed as "Title". I then put that into Google along with "aerialsandtv" to ensure results for my site are brought up. Then I click "Highlight search text" in my Google toolbar and Hey Presto, there it is !
One particularly difficult link to find was where I`d updated a dead link but accidentally I hadn`t highlighted all the link text word, I`d missed the first letter ! Thus that single letter was still pointing to the original broken link. Any reader would have almost certainly clicked on the rest of the word but any search engine would have bought it up as a dead link (plus the updated one obviously). I found it because the Xenu PC full list revealed the link text to be "f", as in just f. Then the penny dropped.....