How do I know if my catalytic converter needs replacing?

Yes. Common signs include a loss of power, reduced fuel economy, a sulfur or rotten-egg smell from the exhaust, a rattling noise under the vehicle, or a check engine light with related codes (like P0420 or P0430). A professional diagnostic is needed to confirm.

The catalytic converter is a key component of your exhaust system, designed to reduce harmful emissions. When it fails or becomes clogged, it can affect performance, emissions, and safety. This article explains how to recognize the signs, how mechanics diagnose the issue, and what to expect if replacement is needed.

Common signs that your catalytic converter may need replacement

The following indicators often point to a failing catalytic converter, though other issues can mimic them. If you notice several of these symptoms, have a technician diagnose the system.

• Check engine light is on, with codes such as P0420/P0430 indicating catalytic converter efficiency problems.


• Noticeable loss of power or sluggish acceleration.


• Lower fuel economy or sudden drop in mpg.


• Strong sulfur or rotten-egg odor coming from the exhaust.


• Rattling or metallic sounds from beneath the car, especially on startup or idle.


• Vehicle fails an emissions test.


• Exhaust system feels unusually hot or the underside is hotter than normal after a drive.

Because these symptoms can also result from issues such as a faulty oxygen sensor, misfiring engine, or exhaust leaks, a professional diagnostic is essential to confirm a catalytic converter problem.

How professionals diagnose a catalytic converter issue

Before deciding on replacement, technicians perform tests to determine if the converter is the root cause. The process combines code reviews with physical checks and performance tests.

• Read trouble codes with an OBD-II scanner and review sensor data, including catalyst efficiency and downstream oxygen sensor readings.


• Inspect the exhaust system for leaks and check for misfires or fuel delivery problems that could damage the converter.


• Perform a catalyst efficiency test or a backpressure test to assess whether the converter is restricting flow or not converting exhaust gas properly.


• Visually inspect the converter for physical damage, melted substrate, or a loose heat shield.

Note that some vehicles require specialized equipment for backpressure or temperature-based tests. If the converter appears intact but efficiency codes persist, replacement may be necessary; if other issues are present, those must be addressed first.

When replacement is necessary and what it involves

Replacement is typically advised when diagnostics indicate the converter is clogged, damaged, or unable to meet emission standards after any underlying issues are fixed.

• Clogged or damaged converter: The most common reason for replacement, especially after prolonged misfires or contaminated fuel.


• Physical damage or overheating: A broken or severely degraded converter can pose safety risks and performance problems.


• Warranty considerations: Some vehicles have emissions warranties that cover catalytic converter failures; check regional regulations and your vehicle’s warranty.

Replacement usually involves removing the old unit and installing a new catalytic converter, with labor typically ranging from 1 to 3 hours depending on vehicle design. You can choose OEM (original equipment) or high-quality aftermarket units, and a qualified technician should ensure proper fit, sealing, and downstream sensor operation.

Cost, options, and what to expect during replacement

Costs vary by vehicle, region, and whether you use OEM or aftermarket parts. Here are general expectations to guide planning.

• Typical total cost range: roughly $900 to $2,500, including parts and labor. Higher-end vehicles or OEM parts can push toward the upper end.


• Parts options: OEM catalytic converters offer exact fit and may come with warranties; aftermarket converters can be more affordable but vary in quality and warranty terms.


• Labor: Most replacements fall within 1–3 hours, depending on exhaust routing, mounting hardware, and access to the converter.

Ask your shop for a written estimate that itemizes parts, labor, and any additional fees. Also request warranty details for the replacement part and the workmanship.

Prevention and maintenance tips to extend catalytic converter life

Taking care of the engine and exhaust system can reduce the risk of premature catalytic converter failure in the future.

• Keep the engine well-tuned to prevent unburned fuel from entering the exhaust, which can overheat and damage the catalyst.


• Address misfires promptly by servicing spark plugs, ignition coils, and fuel injectors.


• Use high-quality fuel and ensure the fuel system is clean and functioning properly.


• Drive for regular longer trips to bring the catalyst up to operating temperature and burn off accumulated contaminants.


• Fix exhaust leaks promptly to prevent oxygen sensor errors and catalytic inefficiency.

Preventive maintenance helps protect the catalytic converter, but if symptoms appear, a professional evaluation is the best path to avoid unnecessary replacement and ensure emissions compliance.

Summary

The catalytic converter plays a vital role in reducing vehicle emissions. If you experience power loss, poor fuel economy, a sulfur smell, a rattling noise, or a check engine light with relevant codes, have a technician diagnose the system. Replacement is typically warranted when the converter is clogged or damaged, after confirming that underlying issues are resolved. Costs vary, so obtain a detailed quote and warranty information. Regular maintenance and mindful driving can extend the life of this important component.

What are the signs that a catalytic converter needs to be replaced?

Signs of a failing catalytic converter include a check engine light, poor engine performance (like sluggish acceleration or misfires), a rotten egg or sulfur smell from the exhaust, and rattling noises from underneath the vehicle. Other indicators are a failed emissions test, reduced fuel economy, and, in severe cases, a glowing red converter due to overheating. 
You can watch this video to learn about the symptoms of a bad catalytic converter: 58s8020 AutomotiveYouTube · Apr 14, 2024
Performance and engine issues

• Sluggish acceleration: The engine may feel weak or have trouble picking up speed, especially when going uphill. 
• Reduced engine power: A general sense of power loss or hesitation can occur. 
• Engine misfires or rough idling: The engine may not run smoothly. 
• Difficulty starting: The engine may take longer to start or have trouble starting at all. 

Smells and sounds 

• Rotten egg smell: The smell of sulfur or rotten eggs coming from the exhaust is a classic sign of a failing converter. 
• Rattling noises: A rattling sound from under the car can mean the honeycomb-like structure inside the converter has broken apart. 
• Black exhaust smoke: Instead of the normal thin white smoke, you might see black smoke coming from the tailpipe. 

This video explains how to diagnose a bad catalytic converter by listening for rattles: 56sClassic Car MaintenanceYouTube · Oct 18, 2025
Other indicators

• Check engine light: The check engine light can be triggered by a malfunctioning catalytic converter. 
• Failed emissions test: A clogged converter will likely cause the vehicle to fail an emissions test. 
• Decreased fuel economy: You may notice you are getting fewer miles per gallon than usual. 
• Overheating: A clogged converter can cause the exhaust to become excessively hot, sometimes even glowing red. 

How do you check for a bad catalytic converter?

Itself next we're going to use our thermometer. And check the bottom temperature of the catalytic. Converter if it's anywhere from 50 to 100°. Under what it was at the top of the catalytic. Converter.

How does a car act when the catalytic converter is going out?

Symptoms of a bad catalytic converter include a sulfur or rotten egg smell, a rattling noise, and reduced engine performance like poor acceleration and low fuel economy. Other signs are a check engine light, failed emissions tests, and difficulty starting the car. If the converter is completely clogged, the engine may stall, as it can't "breathe" properly.
 
Performance and efficiency

• Reduced engine performance: The engine may feel sluggish, hesitate, or have a rough idle. 
• Poor acceleration: The vehicle may struggle to accelerate, especially when going uphill or needing to pass others. 
• Lower fuel economy: You may notice a decrease in your car's miles per gallon. 
• Engine misfires: Incomplete combustion can cause the engine to misfire. 
• Stalling: A severely clogged converter can restrict exhaust flow so much that the engine stalls. 

Smells and sounds 

• Sulfur or rotten egg smell: A properly working converter converts hydrogen sulfide to a harmless gas, but a faulty one will emit a rotten egg odor from the exhaust. 
• Rattling noise: Broken internal components can cause a rattling sound, which may get worse over time. 

Warning signs and diagnostics

• Check engine light: This is often illuminated when the catalytic converter is not working correctly. 
• Failed emissions test: The vehicle will likely fail emissions testing if the converter isn't functioning properly. 
• Discolored housing: A visual inspection may reveal the exterior of the converter is glowing red, indicating a severe clog. 

Can I clean my catalytic converter instead of replacing it?

Catalytic converters cannot be cleaned with any substance. They must be replaced. The most cost-effective option is to have universal ones welded in, which is approximately half the cost of a new pipe and converters.