And only the woofer voice coil at that. Other drivers have a capacitor in series with them so do not show up in an ohmmeter reading. Ohmmeters apply DC and impedance involves AC and the motion of the speaker. And 4 ohm speakers usually measure around 3 - 4 ohms.

All speakers have impedances higher than the resistance you'll measure with an ohmmeter.* The one exception I have seen was a really stooopid design with a bass low-pass filter that was "designed" by ear with no attention paid to its impedance (small series coil, huge capacitor to ground on its input). It smoked a few amps. This was around 1978.

One loose strand of wire touching the paint can short out a Yamaha amp. Check the volume at which the amp shuts off; remove speakers one at a time and suss out whether one particular speaker or speaker wire makes it die. Note that the problem could be at the speaker end. Also find out if anyone moved the receiver between the time it did not shut off and the time it started to shut off.



*I keep repeating "with an ohmmeter" because it is possible to actually measure the real impedance, but not with an ohmmeter, and I want to stress that while an ohmmeter can help you diagnose some problems, its actual information is limited in scope.