Sometimes even the veteran technicians need a little extra help. Consider our story protagonist, Jack McGinnis, head of HiTech Investigations, Inc., working with his partner, Al. He called Jack about a left front door speaker not working on a fairly new SUV.
"All the other speakers work except for the left front door speaker," Al said. "Service information said to unplug the speaker, check for one-half battery voltage at each of the two speaker wires and if the voltage is okay, replace the speaker."
I knew where this one was going. "And then what did you do?"
"Service information said to replace the speaker and I did. But the new speaker didn't work."
"Well, Al, at least you know what it wasn't--it wasn't the speaker. What did you do next?"
"I checked the wires for opens, shorts, and continuity. They all checked out okay, so I replaced the radio. That didn't fix it."
"Well, at least you know what it wasn't," I said.
"Okay, wise guy, if it isn't the radio, the speaker or the two wires, what could it be? I checked everything else. There are no codes, the radio has power and ground, there are no bulletins about the concern, so what is left?"
"Did you check the A/C voltage on the speaker wires, Al?"
"This is warranty work--the manufacturer isn't paying me to check the A/C voltage on the speaker wires."
"Well, Al, you're not getting paid now, are you?"
"Why check A/C voltage?" He asked.
I know you can answer this, dear reader. "Speakers work by cone excursion. The speaker moves in and out. A/C voltage drives the speaker cone. Crank up the volume, the A/C voltage goes up. Does that make sense?"
"Not really," Al said. The diagnostic steps said to check for DC voltage. How would I know that A/C voltage drives the cone?"
"Well, if you read the circuit description paragraph, it mentions the A/C."
"Oh," Al said.
"Let me put it another way, Al. With the speaker disconnected, you're measuring open-circuit voltage."
"Oh," Al said. After a pause, he said "OH!" Much louder. "I know exactly what's wrong with the speaker now."

Do you?