The Part-Number Checklist for Koyo Radiators, Catalytic Converters, and D3S Headlight Bulbs

This Is the Part-Number Checklist I Use Before Ordering Any Aftermarket Part

I'm an office administrator for a 92-person company. I manage about $115,000 a year in purchasing across eight vendors. Some of that is office supplies, but a lot of it is aftermarket auto parts for our fleet vehicles. After five years of this, I've learned that the part number is the single most important piece of information in an order. I can't rebuild a car, but I can buy the right part the first time.

Use this checklist when you're about to buy something described as "direct fit," "OEM replacement," or "compatible with." It works for a Koyo radiator RSX 02-06 part number comparison, a Koyo radiator S2000, a catalytic converter database lookup, or a D3S headlight bulb nearby. If you skip the verification, you're gambling on a return policy.

Step 1: Get the Existing Part's Exact Number Before You Search

The first thing I do is look at the part already on the vehicle. The OEM number is usually stamped on the radiator tank, the converter shell, or a sticker on the headlight housing. I write it down and photograph it. If I can't read it, I use the VIN to get the original part's specifications. With aftermarket parts, exact year and trim are not enough. The VIN is more reliable because some models had mid-year revisions.

I don't trust a product photo. In 2020, I ordered a replacement radiator for a Honda based on the vehicle model, not the part number. The listing showed a radiator that looked exactly like ours. When it arrived, the lower mounting bracket was 1/4 inch wider. It didn't fit. I paid return shipping and waited another ten days. That order cost more than the radiator itself.

Step 2: Koyo Radiator RSX 02-06 Part Number Comparison

The "02-06 RSX" label covers several engine variants. In a Koyo radiator RSX 02-06 part number comparison, the key differences are core thickness, inlet/outlet location, and the type of hose connection the engine uses. The direct-fit version for a street car is not automatically the same as the higher-performance version.

I don't have hard data on every Koyo radiator SKU for this chassis, but from what we've ordered, my sense is that the direct-fit Koyo unit works well for a mostly stock RSX. The thicker-core versions need extra space, and sometimes the A/C lines have to be repositioned slightly. Both are legitimate parts. They're just not interchangeable. If you compare two part numbers, check outside dimensions, core thickness, and inlet/outlet diameters before you decide.

Step 3: Koyo Radiator S2000: Verify the Chassis Code

For the S2000, the same advice applies: don't order by model name alone. The AP1 and AP2 cars use different radiators. A Koyo radiator S2000 for an AP1 may not fit an AP2 without changing hoses. I want to say the main difference is the upper hose connection angle, but don't quote me on that. What I do know is that a good listing should include the chassis code. If it just says "S2000 00-09," that's a red flag. Ask the seller for the exact part number. If they can't give one, move to another vendor.

Step 4: Use a Catalytic Converter Database to Confirm Emissions Compliance

Catalytic converters are the easiest part to get wrong. I always look up the OE reference number in a catalytic converter database before ordering. There are multiple versions with different sensor locations and emissions certifications. A converter that bolts up physically may still fail a visual inspection if it doesn't have the right approval number.

Per FTC guidelines (ftc.gov), product claims have to be truthful and not misleading. So if a supplier says "50-state legal," they should be able to show the EPA/CARB EO number or an exemption notice. If they say "it should pass," I read that as "it won't." (Should mention: the CARB EO number matters most if the car is registered in a state that follows California emissions rules.)

Step 5: D3S Headlight Bulb Nearby: Check the Bulb Code Before You Drive

Searching for a d3s headlight bulb nearby? Auto parts stores often have one in stock, but "nearby" doesn't mean it's the right bulb. D3S and D1S are not the same. Check the bulb's wattage, original equipment equivalent, and color temperature. One 4300K bulb and one 6000K bulb look nothing alike in the same pair.

I almost ordered the wrong D3S bulb in 2023 because it was the last one in stock at a local shop. The catalog said it fit, but the housing needed a different base. The counter person caught it before I paid. That's the kind of save that doesn't happen online.

Step 6: Ask "What's NOT included" Before You Checkout

I've learned to ask "what's NOT included" before "what's the price." The radiator might be listed at a great unit price, then the cap, drain plug, shipping, and core handling get added later. The vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end.

In 2024, I compared two Koyo distributor quotes for the same part number. One was $45 cheaper on the unit price. The other one listed every fee from the start. The surprise wasn't the price difference. It was how much hidden value came with the upfront-fee vendor—clear invoicing, actual support, and no restocking games.

The vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end.

Step 7: Get the Return Policy in Writing (Seriously)

This is the step people skip. Before any order, I find the return policy and copy it into my order notes. I once paid a $60 restocking fee because a converter had a sensor bung in the wrong place. The return policy said 20%, but I hadn't read it. So glad I started checking this first. Almost ordered from a distributor who charges 20% restocking plus return shipping; I would have been out twice as much.

Common Mistake: "Why Do My Outlets Spark When I Plug Something In?"

I'll add a non-car example because it's the same instinct. Why do my outlets spark when I plug something in? A tiny, split-second spark can happen with a worn plug or when you plug it in at an angle. That's usually not a problem. A loud pop, smoke, scorch marks, or a spark that happens every single time means an electrical problem. Stop using the outlet and call an electrician. A part number won't fix bad wiring—and a cheap replacement part won't fix the wrong underlying issue.

People assume the lowest quote means the vendor is more efficient. What they don't see is which costs are hidden or deferred. Same with electrical issues: the first sign may be small, but the problem can be bigger.

Bottom Line

Write down the part number, verify it against a comparison or database, ask what's not included, and get the return policy before you pay. It takes about ten minutes. It saves weeks of returns. I still make mistakes, but the expensive ones stopped when I started using this list every time.

Kavita Iyer

Kavita Iyer is an automotive filtration analyst specializing in engine air filters, oil filters, fuel filters, cabin air filters, and replacement filter elements. She uses ISO 5011, ISO 4548-12, and ISO 19438 methods to evaluate fractional efficiency, contaminant capacity, pressure drop, bypass behavior, seal integrity, and flow restriction. Her work helps distributors, fleet operators, and service networks compare filtration performance, establish replacement intervals, and avoid choices based only on dimensions or marketing claims.