Autolite Iridium Spark Plugs FAQ: 4164, Crankshaft Sensor, Turn Signal Bulbs & Exhaust Pipes

Full disclosure up front: I'm not a mechanic. I'm the office administrator who manages parts purchasing for a service fleet of about 50 vehicles, including two courier motorcycles. That's roughly $90,000 a year in orders spread across six vendors, and I answer to both operations and finance. When I took over purchasing in 2020, I knew almost nothing about the parts side. Five years and a couple of expensive lessons later, these are the questions that come up most often.

If you want the short version of what I've learned: buy the right part once, verify fitment before you order, and never trust a code without checking the wiring. Details below.

• What's actually different about Autolite iridium spark plugs?
• What is the Autolite 4164 spark plug, and will it fit your car?
• How do you know if your crankshaft sensor is bad?
• What should I look for in a rear turn signal bulb?
• Motorcycle exhaust pipe — does it actually matter?
• Should you just buy the cheapest part that fits?

What's actually different about Autolite iridium spark plugs?

The simple version: iridium is significantly harder than platinum and has a much higher melting point. That lets a manufacturer like Autolite build a center electrode as thin as roughly 0.5 mm. A thinner electrode concentrates the spark into a sharper, more consistent ignition event. In everyday terms, that means easier starts, smoother idle, and a longer window before the electrode gap wears out.

But you're probably here for the version that matters to your wallet.

Here's the rough math I did back in 2023 when I moved our work vans from copper plugs to Autolite iridium plugs (labor rates at our shop haven't changed much as of early 2025, at least):

• Copper plugs: $4 per plug × 4 plugs + $120 labor = $136 per change. At a 30K-mile replacement interval, that's three changes over 90K miles — about $408.
• Iridium plugs: $10 per plug × 4 plugs + $120 labor = $160 per change. At a 90K-mile interval, that's one change — $160.

The iridium option wins before you even count the vehicle being off the road. Multiply by 50 vehicles and it stops being a rounding error.

Honestly, I'm not sure why more fleet buyers don't make the same switch. My best guess is the up-front price per plug scares people out of doing the mile-by-mile math. If you ask me, that's exactly backwards — copper is the expensive plug, you just don't notice until the third invoice.

What is the Autolite 4164 spark plug, and will it fit your car?

The 4164 is one of the Autolite part numbers that shows up constantly when I search parts catalogs. It's a mainstream service plug — not entry-level, not a specialty racing part. It gets specified across a range of four-cylinder and six-cylinder applications, which is why it's one of the numbers we keep on the shelf.

Here's the part I want to be straight about: I don't have the heat range and gap spec for the 4164 memorized, and you shouldn't trust anyone who rattles off fitment from memory either. Engine families change mid-generation. A plug that fits a 2016 model can be wrong for the 2018 with the same badge.

What I do before I order any plug number:

1. Check the year, engine code, and VIN — the VIN catches the mid-generation changes.
2. Cross-reference the Autolite application guide with the owner's manual spec.
3. Compare the old plug's reach and electrode design before buying a whole set.

That last step is how I learned that 'same motor, must be the same plug' is a trap. I sent back a full case once (note to self: verify before ordering a case, not after). The 4164 is a solid plug in its applications — but 'in its applications' is doing the heavy lifting. Confirm first.

How do you know if your crankshaft sensor is bad?

Symptoms to watch for: the engine cranks but won't start, it stalls while driving and won't immediately restart, the check engine light registers a crankshaft position sensor code (P0335 or P0336), or the tachometer drops to zero while the engine is clearly still running.

Now the part that costs people money. Those symptoms point at the sensor, but they don't prove the sensor is dead. The code tells you the circuit has an issue. It doesn't tell you whether the sensor itself is at fault, a wire is chafed, or the connector is corroded. A brand-new sensor can trip the same code if the harness is damaged.

I know this because I made the mistake. In 2023, every spreadsheet, symptom check, and code read said 'replace the crankshaft sensor' on one of our vans. My gut said to inspect the wiring harness first — that van lived on gravel roads. I went with the data anyway. After a $140 sensor and an hour of shop time, the van still wouldn't start. Finally checked the harness, found a rubbed-through wire grounding against the engine block, repaired it, done.

The numbers pointed at the sensor. My gut pointed at the wiring. The wiring was right. These days, I spend twenty minutes with a multimeter and a visual harness check before I order anything.

What should I look for in a rear turn signal bulb?

Three things: the right socket type, the right color, and a brand that doesn't oversell you.

Socket types for rear turn signals commonly use 3157, 3156, 7440, or 7443 series numbers. The old bulb or the owner's manual tells you exactly which one you need. Get it wrong and the bulb might physically fit but won't light correctly, or it'll hyperflash and frustrate everyone behind you.

Color matters legally too. In the U.S., a rear turn signal needs to flash amber unless the lens itself is red. That's not a style preference, that's a compliance issue.

On 'long life' claims: I've read too many product listings claiming 50,000 hours from a $3 bulb. Per FTC advertising guidelines (ftc.gov), performance claims have to be substantiated. That rule doesn't stop every listing, but it's a good reason to stick with brands that have been around long enough to be held accountable. A $7 bulb that lasts two years beats a $3 bulb that lasts eight months — especially when the labor to swap it is the same either way.

Over five years of purchasing, the cheap bulbs weren't cheaper. They were just more spreadsheets for me to explain to finance.

Motorcycle exhaust pipe — does it actually matter?

Yes, and I say that with scars. We run two courier motorcycles for fast parts pickups, and the exhaust pipe is not a simple bolt-on costume change.

The pipe changes backpressure and the air-fuel ratio. On a fuel-injected bike that wasn't tuned for it, an aftermarket pipe can lean out the mixture. A lean engine runs hotter, and hot exhaust valves are how you get a $1,500 repair from a $150 pipe.

If you or your company is buying an exhaust for the sound, fine. But budget for the whole project: the pipe, any necessary reflash or tuning, and installation. The cheapest pipe on the market can end up being the most expensive one you ever buy.

In my world, the winning move is a stainless or factory-style replacement that doesn't require re-tuning, from a brand that will still exist when you need a warranty claim. That's not the flashy answer. It's the total-cost answer.

Should you just buy the cheapest part that fits?

No — and that's coming from the person whose boss questions every line item.

Total cost of ownership is the only framework I use when comparing quotes. Add up the part price, shipping, labor, the vehicle's downtime, and the probability that you'll be doing this twice. Suddenly the 'cheap' option doesn't look so cheap.

For spark plugs, that logic points to a quality iridium plug — you do the job once and stop thinking about it until 90K miles. For bulbs, it means buying based on the socket spec and honest specs, not the price tag. For a crankshaft sensor, it means diagnosing before you buy. For an exhaust, it means accounting for the tuning that comes with the pipe.

Personally, I'd rather buy the right part once and hand my finance manager a boring, on-budget report than explain why we're buying the same thing twice.