If you "wipe" the contact by turning the knob back and forth many, many times it will often solve the problems with a rheostat that has some dead spots and also revive those which are DOA. Be patient and just keep turning. Sometimes they are beyond repair but often times after a few hundred "wipes" they work perfectly.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz