In the United States, “cats” is a common short name for catalytic converters. These parts sit in a car’s exhaust system and help change harmful gases into less harmful emissions before they leave the tailpipe. Most modern U.S. cars have one or more catalytic converters because federal emissions rules require them.
What does “cat” mean on a car?
A cat is a catalytic converter. It is an emissions-control device in the exhaust system.
The converter uses a coated ceramic or metal honeycomb. This internal structure has catalyst metals that promote chemical reactions. The converter does not burn fuel or filter dirt like an air filter.
Instead, it changes certain exhaust gases into less harmful compounds.
A typical three-way catalytic converter helps reduce:
- Carbon monoxide (CO) into carbon dioxide (CO₂)
- Unburned hydrocarbons (HC) into carbon dioxide and water
- Nitrogen oxides (NOx) into nitrogen and oxygen
The term “three-way” refers to these three main emissions tasks.
The converter needs high heat to work well. The engine control system uses oxygen sensors to measure exhaust oxygen and adjust the air-fuel mixture. On many gasoline cars, one oxygen sensor sits before the converter and another sits after it. This setup helps the car monitor both engine mixture and converter performance.
Where is the catalytic converter located?
A catalytic converter is installed in the exhaust path between the engine and the muffler. On many cars, it is close to the engine so it can heat up quickly after startup.
The exact layout varies by vehicle:
- A small four-cylinder car may have one main converter.
- A V6 or V8 may use two converters, often one for each bank of cylinders.
- Some vehicles use a close-coupled converter near the engine and another converter farther down the exhaust.
- Hybrid vehicles may use converters that experience different heat cycles because the gasoline engine turns on and off.
You can often identify a converter by its metal housing. It may look like a thick, rounded section of exhaust pipe. Do not rely on appearance alone, however. Heat shields and other exhaust parts can look similar.
Never crawl under a car supported only by a jack. Use properly rated jack stands on a firm surface, or have a qualified technician inspect the exhaust.
How does a catalytic converter work?
A catalytic converter works when hot exhaust passes over its catalyst-coated internal material. The catalyst helps chemical reactions happen faster without being used up in the same way as fuel.
The process depends on the engine sending the converter the right exhaust mixture. If the engine runs too rich, it sends too much fuel into the exhaust. If it runs too lean, the converter may not reduce emissions correctly. A faulty oxygen sensor, leaking fuel injector, worn spark plug, or engine misfire can upset this balance.
The converter can also become damaged by excessive heat. A persistent misfire sends raw fuel into the exhaust. That fuel may ignite inside the converter and melt its internal honeycomb.
This is why replacing a damaged converter without fixing the engine problem often leads to another failure.
What are the signs of a bad catalytic converter?
A failing catalytic converter may cause reduced power, a check engine light, poor fuel economy, rattling, or a sulfur-like smell. These signs can also come from other engine or exhaust faults, so they do not prove that the converter itself has failed.
Common symptoms
Check engine light:
The vehicle may store an emissions code such as P0420 or P0430. These codes usually indicate that the engine computer has detected low converter efficiency. They do not automatically mean the converter is the only problem.
Loss of power:
A melted or blocked converter can restrict exhaust flow. The engine may feel weak, especially during acceleration or at higher speeds.
Rattling noise:
A broken internal honeycomb can rattle inside the converter’s metal shell. A loose heat shield can make a similar noise.
Sulfur or rotten-egg odor:
This smell can occur when the exhaust system is not processing sulfur compounds normally. It can also point to a fuel mixture problem or a converter that is overheating.
Poor fuel economy:
A converter may be involved, but poor mileage more often begins with issues such as low tire pressure, a faulty sensor, a dirty air filter, or an engine problem.
Excessive heat:
A glowing red converter is a serious warning. Stop driving when safe and arrange an inspection. Do not touch the exhaust system, which can remain hot long after the engine stops.
Symptoms and likely causes
| Symptom | Possible cause | What it means |
|---|---|---|
| P0420 or P0430 code | Low measured converter efficiency | Needs diagnosis, not automatic replacement |
| Rattle under the car | Broken converter material or loose heat shield | Inspect the full exhaust |
| Weak acceleration | Restricted converter, ignition fault, fuel problem, or turbo issue | Avoid continued driving if power drops sharply |
| Rotten-egg odor | Converter or mixture problem | Check for overheating and engine faults |
| Red-hot converter | Severe misfire or fuel entering the exhaust | Stop driving and seek professional help |
How do mechanics test a catalytic converter?
A proper diagnosis checks the engine before condemning the converter. A scan code alone is not enough.
A technician may use this sequence:
Scan for trouble codes and freeze-frame data.
Freeze-frame data records conditions present when the fault occurred, such as engine speed and temperature.Check for misfire, fuel, and ignition problems.
Misfire codes, worn spark plugs, damaged coils, and fuel system faults can overheat the converter.Inspect the exhaust system.
The technician looks for leaks, impact damage, loose shields, and signs of overheating.Compare oxygen sensor behavior.
On a functioning system, the upstream sensor usually changes more rapidly as the mixture changes. The downstream sensor should show a more stable pattern. A similar pattern from both sensors may indicate low converter oxygen-storage performance, but sensor or wiring faults can create misleading readings.Test exhaust back pressure or flow when restriction is suspected.
A blocked converter can create excessive back pressure. The test method and acceptable values vary by engine and manufacturer.Check converter temperature when appropriate.
Temperature readings can offer clues, but they must be interpreted with engine load, warm-up time, and sensor accuracy in mind.Confirm the repair.
After repairs, the technician clears codes only when appropriate and completes the required drive cycle. The vehicle may need several trips before the emissions monitors run again.
A code reader can identify stored codes, but it usually cannot prove which part has failed. Diagnosis is especially important because catalytic converters are costly compared with many sensors and ignition parts.
Can you drive with a bad catalytic converter?
You may be able to drive a short distance with some converter faults, but continued driving can cause more damage. The safe choice depends on the symptoms.
Do not continue driving if:
- The check engine light is flashing.
- The engine is shaking or misfiring.
- The converter glows red.
- The vehicle suddenly loses major power.
- You smell strong fuel or burning material.
- Smoke comes from the exhaust or under the car.
A flashing check engine light commonly signals a severe misfire that can damage the converter and other parts. Reduce load, pull over when safe, and arrange service.
A steady check engine light with no severe symptoms may allow limited driving, but emissions performance is not normal. The car may also fail an emissions inspection, depending on state rules.
Why do people steal catalytic converters?
Catalytic converters contain small amounts of valuable metals, including platinum, palladium, and rhodium. Their resale value can make them targets for theft.
Converter theft is more likely when:
- The vehicle has high ground clearance.
- The converter is easy to reach.
- The car is parked outside for long periods.
- The model has a converter that is quick to remove.
- Local scrap markets create demand for stolen parts.
Common signs of theft include:
- A loud exhaust sound
- A harsh roar when starting the engine
- Exhaust fumes under the vehicle
- Missing or cut exhaust sections
- A check engine light
- Sudden loss of low-speed drivability in some vehicles
If you suspect theft, do not place your hands near the exhaust. The vehicle may have sharp metal edges or hot parts. Photograph the damage, contact law enforcement and your insurer, and have the vehicle towed or inspected.
Ways to reduce theft risk
No method guarantees protection, but several steps can make theft harder:
- Park in a well-lit, secure area.
- Use a garage when possible.
- Ask a qualified shop about a converter shield or protective plate made for your vehicle.
- Consider a security marking service where available.
- Set a vehicle alarm to respond to vibration or tilt if your system supports it.
- Check whether comprehensive insurance covers converter theft and what deductible applies.
Do not paint, modify, or weld on an emissions component without checking local rules and the vehicle manufacturer’s requirements. Some modifications can create inspection or legal problems.
How much does a catalytic converter replacement cost?
The price depends on the vehicle, converter location, part type, labor time, and local emissions requirements. There is no reliable single price for every car.
The main cost factors include:
| Cost factor | Why it matters |
|---|---|
| Vehicle year and engine | Different engines use different converter designs |
| Number of converters | Some vehicles require more than one |
| Original equipment or aftermarket part | Parts differ in fit, certification, and warranty |
| Federal or California emissions configuration | The required part may vary by emissions system |
| Rust or broken fasteners | Corrosion can increase labor time |
| Theft damage | Missing pipes, sensors, and shields may need replacement |
| State inspection rules | The replacement may need specific approval |
In the United States, a converter must be suitable for the vehicle’s emissions configuration. A part that physically fits may still be illegal or unsuitable if it lacks the required emissions certification.
For California and other states that follow California emissions rules, check the vehicle label and the part’s approval information. The California Air Resources Board maintains guidance and listings for approved emissions parts. Federal Environmental Protection Agency rules also restrict tampering with or removing emissions-control devices.
Before approving work, ask the shop:
- Which diagnostic evidence shows the converter has failed?
- Is the part approved for this exact vehicle and emissions configuration?
- Does the price include gaskets, hardware, sensors, and labor?
- Will the repair address the cause of the failure?
- What warranty applies to the part and installation?
Can you clean or unclog a catalytic converter?
A severely melted or broken converter cannot be restored with a cleaner. Some products claim to clean deposits, but they cannot rebuild a damaged honeycomb or replace missing catalyst material.
The correct repair depends on the cause:
- A converter with a minor deposit issue may improve when the underlying fuel or ignition problem is fixed.
- A melted converter generally needs replacement.
- A broken internal substrate needs replacement.
- A converter contaminated by coolant, oil, or raw fuel may fail again unless the engine problem is repaired.
- A restricted converter requires testing before replacement.
Avoid removing the converter or installing a hollow shell. In the United States, tampering with required emissions equipment can violate federal law and may prevent the vehicle from passing inspection.
How can you help a catalytic converter last longer?
The best protection is timely engine maintenance. A converter often fails because another problem sends too much fuel, oil, or coolant into the exhaust.
Helpful steps include:
- Repair a flashing check engine light quickly.
- Replace spark plugs and ignition parts at the intervals listed for the vehicle.
- Fix fuel injector, oxygen sensor, and air-fuel mixture faults.
- Repair oil or coolant leaks that enter the combustion chamber.
- Use the correct fuel and engine oil for the vehicle.
- Avoid ignoring rough running or repeated stalling.
- Keep the exhaust system and heat shields secure.
- Follow the maintenance schedule in the owner’s manual.
Do not pour unapproved chemicals into the fuel tank or exhaust. A product may hide a symptom while the engine continues to damage the converter.
Common mistakes to avoid
Replacing the converter based only on a code
P0420 and P0430 are performance codes, not direct proof that the converter is physically broken. Check for exhaust leaks, sensor faults, fuel problems, and misfires first.
Driving with a flashing check engine light
A flashing light usually needs prompt attention. Continued driving can turn a smaller ignition repair into a converter replacement.
Buying the cheapest converter without checking approval
A low-cost part may not meet the emissions standard for the vehicle or state. It may also have the wrong fit or catalyst loading.
Removing the converter to restore power
Removal may create a loud exhaust, trigger warning lights, increase emissions, and violate emissions rules. It can also reduce resale value and cause inspection failure.
Touching a hot converter
Converters can reach very high temperatures. Let the vehicle cool fully before inspection, and use professional equipment when checking temperatures or exhaust flow.
Quick answers about car “cats”
Are cats and catalytic converters the same thing?
Yes. In automotive slang, “cat” or “cats” usually means catalytic converter or catalytic converters.
Do electric cars have catalytic converters?
Fully electric cars do not have tailpipe exhaust, so they do not use catalytic converters. Hybrid and plug-in hybrid vehicles with gasoline engines generally do.
Does every car have the same number of catalytic converters?
No. The number depends on the engine, exhaust layout, model year, and emissions system. A vehicle may have one, two, or more.
Can a bad catalytic converter damage the engine?
A blocked converter can increase exhaust back pressure and reduce engine performance. The original cause, such as a severe misfire, may also damage the engine or exhaust system.
Will a catalytic converter replacement turn off the check engine light?
It may, but only if the converter was the real cause and no other fault remains. The emissions monitors may need a complete drive cycle before the system confirms the repair.
Final takeaway
“Cats” on a car are catalytic converters. They reduce harmful exhaust emissions by converting carbon monoxide, unburned hydrocarbons, and nitrogen oxides into less harmful gases.
A check engine code does not prove that the converter needs replacement. Have the engine, sensors, exhaust leaks, and converter tested as a system. If the light is flashing, the engine is misfiring, or the converter is glowing, stop driving when safe and get professional help.
