I’ve worked as an ASE-certified automotive technician for over a decade, and every so often, a phrase shows up on cars that confuses people: “No Cats.” Friends send me photos. Customers ask about it while they’re in for an oil change. And occasionally a car rolls into my bay with the words written right on the underbody in significant marker.
“Cats” in this context doesn’t mean animals. It means catalytic converters.
A catalytic converter is part of the exhaust system. Its job is to reduce harmful emissions before they leave the tailpipe. When someone writes “No Cats” on a car, it almost always means one of two things:
- The catalytic converters have been removed
- The owner wants potential thieves to know there’s nothing left to steal
Both situations occur regularly in my shop, and they have very different backstories.
What I actually see in the shop when a car has “no cats.”
The first time I saw the phrase written across a muffler shield, the car had been towed in. The owner was driving on the highway when the check engine light came on, and the vehicle suddenly became much louder. Once it was on the lift, it was apparent: the catalytic converters had already been removed, and a straight-pipe section. Someone had been installed. Someone later added “NO CATS” in big letters underneath — likely as a warning to thieves who scout for converters from underneath vehicles.
The car ran, but not well. The exhaust smell was harsh, the oxygen sensor readings were out of range, and the engine management system was constantly trying to compensate. The owner hadn’t removed the converters themselves; they had bought the car used and didn’t realize what “no cats” had implied on the bill of sale. That situation is more common than people expect.
I’ve had the second kind of case too — a truck owner who deliberately removed their catalytic converters for “more sound and power.” They later admitted they had underestimated the roughness of daily driving without them. They also had trouble passing inspection, couldn’t register the vehicle, and eventually paid to have converters reinstalled. They told me afterward that the few minutes of loud exhaust weren’t worth the hassle or the expense.
Why do people write “No Cats” on the car itself
Most of the time, it’s about theft deterrence.
Catalytic converter theft has been a real problem in recent years. Thieves look for the precious metals inside the converters and can spot which vehicles typically have larger ones. Some owners spray-paint the exhaust bright colors or write “NO CAT” or “NO CATS” underneath to signal: there’s nothing valuable left here.
From what I see in the shop, this marking does two things:
- It warns thieves that they’ll be wasting their time
- It alerts future mechanics or buyers that the emissions system has been altered
Whether it truly stops theft attempts is hit-or-miss. I’ve seen vehicles marked “NO CAT” that were still damaged underneath by someone who didn’t bother to look closely first.
What it actually means for the car mechanically
Driving without catalytic converters changes more than people think.
Engines are managed by sensors located before and after the converter. When converters are removed, the readings change, and the car’s computer tries to “correct” a problem it thinks exists. In my experience, this often leads to rough idle, poor fuel economy, persistent check engine lights, and that raw exhaust smell drivers complain about after the modification.
There’s also the noise. People expect “sporty.” What they usually describe later is “boomy and tiring on long drives.”
On top of that, I’ve seen insurance claims denied, and vehicles fail routine inspections simply because the emissions system was tampered with. Reinstalling converters afterward is rarely cheap, especially on vehicles with multiple banks.
My professional opinion is straightforward: removing catalytic converters on a street-driven car is usually a bad idea. It creates legal, drivability, and resale problems. I’ve never had a customer come back years later, glad they did it.
If you’re thinking of buying a car marked “No Cats.”
This situation walks into my shop a few times a year. Someone finds what looks like a great deal online, notices “no cats” in the description or sees the words written under the vehicle, and wants to know if it’s safe to buy.
Here’s what I typically look for during inspection:
- whether converters have been physically removed or replaced with pipes,
- whether the engine computer has been altered to hide warning lights
- The oxygen sensor wiring has been bypassed or tampered with. Failed or incomplete emissions readiness monitors
I’m not giving you a checklist to do this yourself — I’m saying, from experience, that this is best left to someone who sees it often. More than once, a buyer assumed it was “just a sticker thing” and ended up with a car they couldn’t legally register in their home state.
If you see “No Cats” in an ad or on the car, treat it as a bright yellow caution flag and have it inspected by a qualified technician before making a decision.
What I advise as a working technician
My advice is shaped by what I see on the lift, not by theory.
If you already own a vehicle marked “No Cats,” have it inspected. Understand whether converters are missing and what it would take to restore the emissions system to factory configuration. People are often relieved to know where they stand.
If you’re writing “NO CATS” under your car purely as theft deterrence but still have converters installed, be aware that you may accidentally signal the opposite to a buyer or inspector later. I’ve had awkward conversations in both directions.
And if you’re considering removing catalytic converters altogether, I strongly advise against it for any road-driven vehicle. The headaches show up later — in drivability, legal trouble, and resale value — and I’m usually the one you see after those headaches arrive.
The phrase looks simple, but it usually points to a story underneath the car. I’ve learned that a quick look from above rarely tells that story. A lift, a light, and a bit of experience usually do.