If you suspect a resistor, examine it visually first. Is it scorched or burned
looking? Does it have obvious chips or cracks? Is it soldered firmly in place or
does it feel a little loose when you wiggle it a little?
Resistors can fail in the following ways:
- Their resistance may drift from it's nominal value, sometimes dramatically.
Measure the resistor with an multimeter set for ohms and see if it's within tolerance.
These things make a resistor more likely to have drifted in value: high nominal value,
heat stress (near a hot component or gets hot from the current it carries), high voltage
across it (plate resistors).
- They may be completely open (ohmmeter test!)
- They may be mechanically damaged: cracked or chipped, or the leads may be
loose on the body or the leads may be broken themselves or poorly soldered.