Where to find the code on a catalytic converter (and how to read a rusty one)

1. Canister code first — not the shield

Every brand stamps codes differently. Some converters still have a heat shield. Some don’t. Plenty of units show two different codes: one on the shield, one on the canister itself.

Rule of thumb: always use the code on the canister. It may be hiding under the shield. Only use the shield stamp if there is no code on the canister.

• Peel or lift the shield enough to see the body of the cat when it’s safe to do so.
• Don’t assume the shield number matches the canister number — they often don’t.
• If you only grab the easy shield stamp, you can look up the wrong unit and overpay.

2. Rusty codes are normal — clean and light them

Rusty stamps are part of the business. How readable they are depends a lot on where the cars lived. From floor experience, a dry region like Washington can have roughly 90% of cats with a visible code, while a salt-and-moisture region like Massachusetts can run closer to 80% rusted-out stamps. Your mix will follow your market.

When the stamp is hard to see:

• Use your phone flashlight and change the angle — side light often pops characters that look blank head-on.
• Use a hard metal brush to knock crust off the stamp area.
• A light WD-40 / penetrant spray helps loosen rust so the brush can clear the digits. Wipe excess so you can still see the metal.
• If part of the code is gone, don’t invent the missing characters — look up what you can still read.

3. Type the last 4 digits first

When you search a code in an app, the last four characters are usually the useful ones. Those endings tend to differ from cat to cat. The first few characters often match a whole family of units and flood you with lookalikes.

Example from the floor: many General Motors converters start with 12 or 25. Typing only that front piece brings a long list. Typing a clear ending narrows it fast.

That’s why partial search matters. If the front of the stamp is rusted out but the last four are clean, start there, then confirm the match by shape, brand family, and what’s left of the stamp before you pay.

4. No code left? Don’t guess a stamp

If there is no readable code — fully rusted, cut away, or never stamped clearly — do not invent a full number just to get a high list price.

A safer approach: use a national price average as a cautious reference for that kind of unit (CatXapp shows a National Price Average on the home screen), then adjust for condition, integrity, and your margin. Still verify with your own eyes. Empty, aftermarket, or damaged units are not average OE money.

5. Tools that actually get used on the floor

• Gloves — cut converters have sharp metal. Cuts are common without them.
• Phone flashlight — always handy for angles on worn stamps.
• Hard metal brush — clears rust so you can read what’s left.
• Penetrant spray — loosens rust before brushing.
• Partial code search — when the stamp is only half there, look up the piece you trust.