Can a car run without a catalytic converter?

Yes, most cars will physically run without a catalytic converter, but doing so is illegal in many places, much louder, significantly more polluting, likely to trigger a check-engine light and fail inspection, and can expose you to fines and insurance or warranty problems. Below, we explain how the car behaves mechanically without the converter, the legal and environmental stakes, and what to do if yours is missing or stolen.

How engines behave without a catalytic converter

The catalytic converter sits in the exhaust stream, cleaning up carbon monoxide, unburned hydrocarbons, and nitrogen oxides before they exit the tailpipe. Removing it doesn’t stop the engine from running; it changes the exhaust flow and the way the engine computer monitors emissions.

Modern cars use an oxygen sensor before the converter to control fuel mixture and a sensor after the converter to confirm the catalyst’s efficiency. Without the converter (or if the downstream sensor sees abnormal readings), the car will usually log faults (such as P0420/P0430), illuminate the check-engine light, and set emissions monitors to “not ready.” Many vehicles still drive normally; some may alter fueling strategies or exhibit minor drivability quirks due to exhaust leaks or sensor placement.

Performance, noise, and fuel economy

With the converter gone, exhaust noise jumps dramatically—often to illegal levels—because there’s less muffling. Backpressure generally drops, which rarely delivers meaningful power gains on modern naturally aspirated engines; on turbocharged setups, spool characteristics may change slightly. Fuel economy can worsen if the engine falls back to less precise control strategies or if leaks skew sensor readings. Mechanically, running without a converter usually doesn’t damage the engine by itself, but exhaust leaks, overheated wiring, or prolonged rich/lean operation caused by sensor issues can lead to longer-term problems.

Legal and regulatory landscape

In the United States, the federal Clean Air Act prohibits removing or tampering with emissions equipment on roadgoing vehicles, regardless of state. Many states add inspection checks: vehicles typically fail if the catalyst monitor isn’t “ready” or if a visual inspection finds the converter missing. Penalties can reach thousands of dollars per vehicle, and shops face even stiffer fines for performing “cat deletes.” Similar rules apply in Canada (provincial inspections), the UK (MOT failure), and across the EU and Australia (roadworthiness and emissions regulations). “Race-only” or off‑road exceptions are narrow, often contested, and do not generally apply to vehicles originally built for the street.

Risks and side effects of running without a catalytic converter

The points below outline the practical consequences drivers typically experience when operating a vehicle without its catalytic converter.

• Legal exposure: Federal and local fines, inspection failure, and potential registration denial.
• Environmental impact: Dramatically higher emissions of CO, HC, and NOx, contributing to smog and health risks.
• Check-engine light and diagnostics: Persistent fault codes (e.g., P0420/P0430) and “not ready” OBD monitors prevent passing emissions tests.
• Noise: Noticeably louder exhaust that can violate noise ordinances.
• Insurance and warranty: Claims may be denied if a loss is tied to illegal modifications; warranties on related systems can be voided.
• Potential drivability concerns: Exhaust leaks or sensor relocation can cause rough running, fuel trim issues, or heat damage to wiring.

Taken together, these factors make driving without a catalytic converter impractical for public-road use, even if the vehicle technically runs.

Diesel and hybrid specifics

Diesel vehicles layer emissions systems—oxidation catalysts, diesel particulate filters (DPF), and often selective catalytic reduction (SCR). Removing any of these usually triggers warnings or limp mode, increases soot and NOx, and is illegal. Hybrids can still run without a converter, but they will fail diagnostics and inspections just like conventional vehicles; some may be more sensitive to exhaust sensor faults due to tight emissions strategies.

If your catalytic converter is missing or stolen: what to do

Converter theft remains common. If you discover yours is gone—often signaled by a sudden, harsh exhaust note—take the following steps to protect your vehicle and stay within the law.

1. Document and report: Photograph the damage and file a police report; you’ll likely need it for insurance.
2. Contact your insurer: Comprehensive coverage may pay for OEM or approved replacement and related repairs (sensors, pipes, shields).
3. Avoid driving far: Short moves to a repair shop are usually safe if there’s no dangling hardware, but the car will be loud and emit pollutants.
4. Use compliant parts: In California and other CARB states, you must install a CARB-approved converter; elsewhere, follow local standards.
5. Protect against repeat theft: Consider an anti-theft shield, updated fasteners, parking in well-lit or secure areas, and marking/etching the converter.
6. Verify readiness: After repair, ensure no fault codes remain and that OBD monitors (including the catalyst monitor) are set to “ready” before inspection.

Following these steps helps restore compliance, enables insurance support, and reduces the risk of another theft.

Cost and repair considerations

Replacement costs vary widely. Many mainstream gasoline cars see total bills around $800–$2,500; certain models—especially hybrids and some luxury vehicles—can run much higher. OEM units tend to last longer and meet certification requirements everywhere; aftermarket units may be cheaper but aren’t legal in CARB states unless specifically approved. Add potential costs for oxygen sensors, exhaust piping, heat shielding, and an anti-theft plate. Comprehensive insurance often covers theft after the deductible.

Bottom line

You can get a car to run without a catalytic converter, but doing so on public roads is illegal in most jurisdictions, dirties the air, and invites inspections failures and fines. If your converter is missing—whether by choice or theft—the practical, legal, and environmental path is to restore a compliant system promptly.

Summary

Mechanically, a car will usually operate without its catalytic converter; legally and environmentally, it shouldn’t. Expect a check-engine light, loud exhaust, failed inspections, and potential fines. Replace the converter with a compliant unit, verify OBD readiness, and consider theft-prevention measures to stay road-legal and protect your vehicle.

How does a car sound without a catalytic converter?

A car without a catalytic converter will produce a much louder, rumbling, or roaring engine sound, similar to a racecar, as the component is partially a muffler and a bottleneck in the exhaust system. The lack of the converter creates less backpressure, allowing exhaust gases to escape with less obstruction, resulting in increased volume and a rougher, sputtering sound, particularly when accelerating. 
This video demonstrates the difference in sound between a car with and without a catalytic converter: 47sAmy SahadiYouTube · Oct 17, 2023
What you’ll hear:

• Loud rumbling or roaring: The engine will sound significantly louder, with a distinct roar that is especially noticeable when the car is started or when you accelerate. 
• Rougher driving: The engine may also feel like it’s driving more roughly, with a sputtering sensation during acceleration, due to the unregulated exhaust flow. 
• Hollow sound: The sound can become hollow and resonate more due to less dense metal in the exhaust pipe where the converter was. 

Why this happens:

• Partial muffler: Opens in new tabA catalytic converter helps to muffle the noise produced by the engine, so its absence results in a louder exhaust sound. 
• Reduced backpressure: Opens in new tabThe converter is a bottleneck in the exhaust system that creates backpressure, and removing it allows exhaust gases to exit with less resistance, increasing the sound volume. 

You can watch this video to learn more about how removing the catalytic converter can affect the sound of the engine: 58sMotor MatchupsYouTube · Jul 23, 2025
Other consequences: 

• Increased emissions: The primary function of the catalytic converter is to reduce harmful emissions. Without it, your car will produce more pollution.
• Check engine light: The absence of the catalytic converter can trigger the check engine light on your dashboard.
• Legal issues: Driving without a catalytic converter is illegal in many places, as it violates environmental and emissions laws.

What are the symptoms of no catalytic converter?

Signs of a missing catalytic converter include an extremely loud or roaring exhaust, particularly at startup and when accelerating, poor engine performance and sluggish acceleration, a check engine light on the dashboard, a sulfur or rotten egg smell from the exhaust fumes, and physical damage or missing components under the vehicle. You may also experience reduced fuel economy and a rougher ride.
 
Audible Signs 

• Loud Roaring Exhaust: The most immediate sign of a missing catalytic converter is a sudden, very loud, roaring, or rumbling sound from the exhaust, especially when starting the engine and pressing the gas pedal.
• Sputtering: The vehicle might make a sputtering sound as you change speed.

Performance Issues

• Decreased Performance: Your vehicle may struggle to accelerate or feel sluggish. 
• Reduced Fuel Economy: The engine may not run efficiently, leading to worse miles per gallon. 
• Rough Driving: You might notice a less smooth and more jarring ride. 

Visual & Sensory Clues

• Smell of Sulfur: A strong sulfur or rotten egg smell, particularly strong inside the vehicle when the windows are down, can indicate an issue with the exhaust system. 
• Check Engine Light: The light will likely illuminate on your dashboard due to the change in emissions and engine sensor readings. 
• Visible Damage: You may see signs of tools used by thieves, such as cuts or damage, to the exhaust system or the undercarriage of the vehicle. 

What to Do

• Inspect Your Vehicle: Carefully check under your vehicle for the missing catalytic converter and any signs of damage. 
• Get It Checked: Have the vehicle inspected by a mechanic to diagnose the problem and arrange for replacement to restore your vehicle’s performance and safety. 

Can you drive after a catalytic converter is removed?

Driving without a catalytic converter could put the health and safety of your passengers at risk. It could also damage your car and land you in legal hot water. Federal law says if your vehicle was equipped with a catalytic converter when it was made, you aren’t allowed to remove it.

How long can you go without replacing a catalytic converter?

You should replace your catalytic converter as soon as possible if it is failing, as a completely failed converter can damage your engine and cause a severe power loss. While a converter can last many years, a faulty unit poses risks to your car and the environment, and you should not continue to drive with a bad converter indefinitely. 
Why you shouldn’t delay replacement

• Engine damage: A clogged catalytic converter can cause serious engine failure over time. 
• Performance loss: You will experience reduced engine power and poor fuel efficiency. 
• Legal issues: Driving without a functioning catalytic converter is illegal in many places due to its role in reducing harmful emissions. 
• Emissions failure: You’ll likely fail an emissions test. 

Signs of a failing catalytic converter 

• A check engine light or a P0420 error code.
• A sulfur (rotten egg) smell from the exhaust.
• Reduced engine performance or difficulty accelerating.
• A rattling sound from under the vehicle.

What to do next

• Get it diagnosed immediately: Visit a qualified mechanic to have the catalytic converter inspected. 
• Don’t wait: If the converter is damaged, failing, or clogged, it needs to be replaced without delay to prevent further issues.