Autolite Spark Plugs FAQ: Double Platinum, Gap Chart, Bad Catalytic Converter Signs and Control Arm Symptoms

You've got spark plug questions, a headlight that won't cooperate, and maybe a check engine light. I'm a quality inspector at Autolite, and I review ignition components before they ship — roughly half a million parts a year. I've been in automotive quality for about a decade. Here are the answers I'd give a friend, including a few that don't sell parts.

Are Autolite double platinum spark plugs worth the extra cost?

Double platinum spark plugs have a platinum pad on the firing surface of both the center electrode and the ground electrode — or rather, that's exactly what makes them "double." In waste-spark ignition systems, the spark travels from center to ground in one cylinder and ground to center in its paired cylinder. Every firing erodes whichever electrode the spark jumps from. Platinum slows that erosion dramatically, which is why Autolite double platinum plugs commonly run 60,000 to 100,000 miles in applications designed for them.

But it's tempting to think "double platinum = better for every car." That's not how it works. If your engine wasn't designed for waste-spark ignition, the second platinum pad is doing less work. And if the original spec was a copper plug, upgrading to platinum won't fix a misfire that comes from a failing coil or a vacuum leak. It'll just be a more expensive part in an engine that still runs poorly.

In our Q3 2024 durability testing, double platinum plugs held their gap noticeably longer than copper across the samples we pulled. The upfront cost is higher. The cost per mile is usually lower. Check your owner's manual first — the right part for the application always beats the premium part in the wrong one.

How do I read the Autolite spark plugs gap chart?

Spark plug gap is the distance between the center electrode and the ground electrode, measured in thousandths of an inch — 0.044", for example. The Autolite spark plugs gap chart on autolite.com lets you enter your vehicle's year, make, model, and engine, then returns the correct plug and the OE gap spec. As of January 2025, it covers most passenger vehicles sold in North America over the last three decades.

Why does the exact gap matter? Set it too wide and the ignition coil has to work harder to jump the distance — you'll feel a misfire under load. Set it too narrow and the spark is weak, which shows up as rough idle and worse fuel economy. We're talking about a difference of 0.010" or less. There is no room for "close enough."

One thing I should add: the plug in the box is not always pre-gapped for your vehicle, even if the part number matches. We set gaps at the factory to a nominal value, but one plug number can fit several applications with different required gaps. Use a wire-style gap tool before installing. Adjust by bending the ground electrode — never the center electrode. If it's off by more than about 0.005", correct it before installation.

This guidance is for stock ignition systems. If your engine has forced induction, an aftermarket ECU tune, or runs on alternative fuel, you're outside what a gap chart can accurately tell you. A dyno session is the right way to dial in the gap for a modified engine.

What are the signs a catalytic converter is bad?

The usual signs a catalytic converter is bad include a P0420 code (catalyst efficiency below threshold), a rattling sound from under the car, a rotten egg smell, sluggish acceleration, and fuel economy that keeps getting worse. If the ceramic honeycomb breaks apart, you can also feel a power loss that seems like a plugged exhaust.

Here's what surprises most owners: a lot of failed converters are the victim, not the culprit. Worn spark plugs cause a misfire, the misfire sends unburned fuel into the exhaust, and the converter overheats trying to burn it off. The converter dies. Replace it without fixing the ignition problem, and you'll be doing the same job again a few thousand miles later.

One more thing worth checking before you spend money: under EPA emissions warranty requirements (40 CFR Part 85), the catalytic converter on most certified passenger vehicles is covered for 8 years or 80,000 miles, whichever comes first. That's federal law, so it doesn't matter which brand the converter is.

I don't run an exhaust shop — I inspect ignition parts. But I've reviewed enough warranty returns over the years to trust the pattern: ignition failure kills catalytic converters far more often than the converter fails on its own. Check the plugs and coils before you buy a cat.

How do I know if my lower control arm is bad?

A bad lower control arm usually clunks over bumps, lets the steering wander, vibrates at highway speed, and wears the inside or outside edge of the front tires unevenly. The control arm is the link between the wheel hub and the vehicle frame. Its bushings and ball joint are what keep the wheel from moving where it shouldn't.

Control arms aren't a product line I inspect at Autolite day to day. But I've been in automotive quality for a decade, and I've seen enough warranty data on suspension parts to recognize the pattern: the rubber bushing tears, the metal sleeve shifts, and the ball joint develops play. Checking it properly requires a lift, a pry bar, and someone who knows how to read what the bar tells them.

Suspension work is a specialist's job, and I'll say that plainly. If you suspect a bad lower control arm, don't keep driving it around for a week hoping it's something else. A ball joint that lets go at speed is a crash. Get a real inspection.

Should I repair my headlight or replace it?

It depends on what's wrong. If the lens is yellowed or cloudy from UV exposure, headlight repair with a restoration kit is a legitimate fix. About $20-40 and an hour of wet sanding, polishing, and UV sealant will get you two or three more years of clear output. Nighttime visibility improves immediately.

If you see condensation inside the lens, stop there. Moisture inside the housing means the seal failed, and no amount of exterior polishing will fix that. You're looking at replacing the assembly or having a shop properly reseal it. The replacement is the standard route; the reseal is the budget route if the housing itself is still intact.

One QA habit to carry with you: when you swap a halogen bulb, touch only the metal base. Skin oil on the quartz glass creates a hot spot, and the bulb can fail in a few thousand miles. I've seen returned bulbs with the fingerprint pattern burned right into the glass. Also: if you crack a housing tab during reassembly, that $15 bulb swap becomes a $300 assembly replacement. Clean glass, clean hands, patience.

Why won't you just tell me the exact spark plug gap for my car?

Because the correct gap depends on the year, engine code, and sometimes even the emissions package of your specific vehicle. Two cars with the same name on the trunk can call for different gaps. If I give you a number from memory and it's wrong, you get a misfire, a rough idle, and an afternoon of frustration that we probably both regret.

That's why the Autolite spark plugs gap chart exists. Enter your vehicle, get the part and the OE gap. The specification in your owner's manual overrides anything I say. From the inspection side, I can tell you the gap is a precision spec, not a suggestion. A plug off by 0.010" can be the difference between a smooth idle and a check engine light.

Some people read that as me dodging the question. I'm not. Confirming a spec instead of assuming it is the discipline that keeps bad parts out of cars. Parts quality and application matching are two different things — I can talk about how a plug is built all day, but I can't tell you what your specific engine needs until I look it up. That's the boundary, and I'm comfortable with it.