Why That Spark Plug and Your Smart Switch Have More in Common Than You Think

So, the project started simple enough. Replace a tired, buzzing single-pole switch in the hallway with a shiny new Leviton Decora Smart Switch 2nd Gen. My brother-in-law, a man of considerable confidence and questionable foresight, had already watched the videos. He had the WiFi smart switch out of the box, the little blue wire nuts laid out on the carpet, and the Leviton app downloaded to his phone. He was ready.

“It’s just wiring,” he said, scrolling through a blurry screenshot of the diagram he’d taken from his laptop. “White to white, black to black, green to ground. It’s done in ten minutes.”

I’ve been a quality and brand compliance manager for about five years now. I review what we deliver—roughly 200 unique items a year—for a company that makes critical electrical components. I’ve seen what happens when “ten minutes” turns into a weekend of confusion and a fried neutral line. I wanted to say, “Hold on, have you read the instructions for that specific Leviton 4-way switch wiring? Are you sure that’s the correct diagram?” But honestly, it was Saturday, I was holding a beer, and I didn’t want to be that guy.

I should have.

The Hallway of Horrors

He turned the power off at the breaker—good start. He pulled the old switch out. Then he stared at the back of the new Leviton smart switch. He stared at the wires in the wall. He stared back at the phone. It was a 4-way switch setup, something he hadn’t anticipated. His “ten minutes” was already shot.

“I think the traveler wires go here?” he muttered, pointing a screwdriver at one of the terminals.

I put my beer down. “You don't think. You verify. Which specific wiring diagram are you looking at? The standard 3-way, or the 4-way diagram with the smart switch as the load?”

The problem wasn't that the instructions were bad—Leviton wiring instructions are actually pretty solid, usually with clear diagrams for 3-way, 4-way, and single-pole setups. The problem was that he was assuming one piece of general knowledge applied to a specific, slightly more complex situation. He was treating the smart switch like a dumb switch with an app. In my line of work, I’d call that a spec mismatch.

Actually, I’d call it more than that. If I could redo that moment, I'd have him pause the whole thing. Looking back, I should have pulled the Leviton 4 way switch wiring instructions up on my own phone. At the time, I didn't want to step on his toes. But in quality, that hesitation is expensive. On a large project, that pause costs a $22,000 redo. Here, it was just about a working light fixture, but the principle is the same.

From Wires to Pistons: The Spec Verification Trap

The next day, he was messing with his old lawn mower. He needed a new spark plug. He had the old one out—a Champion something—and was about to drive to the auto parts store.

“I’ll just get the same one,” he said.

“But you don’t know if that was the right one,” I said, remembering his hallway adventure. “Check the manual.”

He grumbled but looked it up. The manual called for an NGK BPR6ES. I don’t have hard data on how many people just buy the same plug without checking, but based on my experience, that's probably 80% of the DIY world. The BPR6ES is a specific heat range, a specific gap design. The plug he pulled out was a different heat range entirely. The previous owner had just used whatever was on the shelf.

“Will it run?” he asked.

“It’ll run,” I said. “But it might run hot, or foul out easily. It might carbon up faster. The NGK BPR6ES is the spec the engine was designed to use. The old one was a workaround—or an error.”

A Tale of Two Specs

The situation perfectly mirrored the Leviton Decora Smart Switch 2nd Gen install. He had a spec (the NGK plug, the correct wiring diagram) and a reality (the old plug, the confusing 4-way setup). In quality, we call this a non-conformance. He was trying to find a solution that worked, rather than a solution that was correct.

This worked for my brother-in-law, but his situation was purely recreational. At work, if you’re specifying a J17LM spark plug for a small engine fleet because it was “the one that was in there,” and the OEM spec sheet says NGK BPR6ES, you’re inviting a systematic performance problem. Your data will be bad, your engine life will be variable, and you won’t know why until you audit the spec.

I’m not 100% sure, but I think this is why so many small engine service shops stick to one brand. They know the NGK BPR6ES is a reference point. It’s their “Leviton switch” level of standard. The trust in the item comes from consistency.

How to Check Car Battery Voltage with a Multimeter: The Final Judgment

Later that week, his car wouldn't start. Click-click-click. He suspected the battery. I grabbed my multimeter. He was about to just go buy another battery, maybe the cheapest one with the right terminal layout.

“Let’s diagnose first,” I said. “You learned nothing from the switches and the lawn mower?”

I showed him how to check car battery voltage with a multimeter. First, the static voltage: 12.4 volts.

“That’s not great,” I said. “A fully charged battery should be 12.6 to 12.8. But 12.4 means it’s about 75%. It’s low, but it shouldn't be dead.”

Then I showed him the critical test. We cranked the engine while watching the voltage drop. It fell to 9.8 volts while cranking.

“That’s a failure,” I said. “A healthy battery holds above 10 volts under cranking load. The spec is clear—below 10, the battery is sulfated or failing. It’s not the NGK BPR6ES for your small engine; it’s your battery.”

He looked at me. “So I do need a new one.”

“You need to check the spec for your car’s required Cold Cranking Amps (CCA). Putting in the wrong CCA battery is like putting a J17LM spark plug in a motor designed for a NGK BPR6ES. It will work for a while, and then you’ll chase a problem that isn’t there.”

I thought about our Q1 quality audit. We rejected 16% of first deliveries last year because of spec non-conformance. The vendors claimed it was ‘within industry standard.’ It wasn’t. We had the spec. They changed the plug, changed the switch diagram, and suddenly, everything worked.

The Lesson: Trust the Spec, Not the Memory

So what did I learn from that weekend of fixing my brother-in-law’s entire existence?

The first lesson is that a Leviton switch needs the right diagram as much as a NGK BPR6ES needs the right gap. It’s not just “a switch.” The decora smart switch 2nd gen is a powerful computer. If you don’t treat the 4-way switch wiring instructions as gospel, you’re going to end up with a network error or a melted neutral.

The second lesson is that “how to check car battery voltage with a multimeter” is the same process as checking a spec against a delivered part. You measure the state, you compare it to the defined requirement (12.6V static, >10V cranking, or CCA rating), and you make a binary decision. The multimeter is the truth. The spark plug cross-reference is the truth. The Leviton wiring diagram is the truth.

If I could redo that decision to stay quiet, I'd speak up sooner. But given what I knew then—that he’s stubborn—it was a calculated risk. The end result was a well-lit hallway, a starting lawnmower, and a running car. But it took three days and a lot of explaining. In a business setting, that’s a $22,000 delay. In a garage, it’s just a weekend.

Take it from someone who reviews 200 spec items a year: the time you save by guessing is never worth the cost of the rework. Check the spec. Use the right diagram. And for heaven's sake, don’t assume the old plug is the right plug.