Yes the resistor pack is the first thing to check. It's an easy fix I've done it once. Right now my 2 setting doesn't work and I'm waiting for more to go before I replace it again. The resistor pack is just that, four resistors, one for each setting on a small plug-in circuit board. For what ever reason, electrical or mechanical, one by one each resistor pooches out.

Ref: http://en.wikipedia.org/wiki/Resistor