What are the signs of a catalytic converter going bad?

The most common signs include a dashboard check engine light, reduced power and fuel economy, a sulfur-like rotten-egg odor, rattling noises under the vehicle, and emissions test failures. If you notice any of these symptoms, have your exhaust and emissions system checked by a qualified mechanic as soon as possible.

Dashboard signals and diagnostic codes

These are the indicators most drivers first notice, often tied to onboard diagnostics and code reporting.

• Check Engine Light (or Service Engine Soon) illuminated on the dashboard.


• Diagnostic trouble codes commonly associated with catalyst efficiency, such as P0420 or P0430, or codes related to downstream oxygen sensors.


• Emissions-related test failures or incomplete readiness monitors on older vehicles.

These indicators typically prompt a code readout and further testing to determine whether the catalytic converter is at fault.

Performance and drivability indicators

Beyond lights and codes, a failing converter often affects how the car runs and responds while driving.

• Loss of power or sluggish acceleration, particularly at mid-range RPMs where the converter’s efficiency matters.


• Reduced fuel economy due to increased exhaust backpressure and inefficient exhaust treatment.


• Engine misfires or rough idle, which can occur if the converter becomes clogged or the engine runs poorly.

These symptoms reflect the converter’s reduced ability to process exhaust, which can strain the engine and fuel system over time.

Audible and sensory cues

Certain sounds and smells can point toward catalytic converter issues, especially when the system is deteriorating or damaged.

• Rattling or banging noises from under the vehicle, often indicating a broken ceramic substrate or a loose internal component.


• Rotten-egg sulfur smell near the exhaust, which can signal catalyst problems or overheating.


• In some cases, unusual exhaust smoke, though this is less common and may indicate other engine problems as well.

Audible and olfactory clues should be evaluated promptly to prevent further damage to the exhaust system or engine.

Diagnostics and testing steps

Mechanics use a combination of tests to verify converter performance and distinguish it from related issues.

• Live data monitoring with an OBD-II scanner, focusing on downstream oxygen sensor readings and comparing them with upstream sensors.


• Exhaust backpressure testing or temperature measurements across the converter to assess flow and heat generation.


• Visual inspection for external damage, heat shield integrity, and signs of substrate breakage or clogging inside the converter.

Correct diagnosis helps determine whether the converter itself is failing or if another component—such as an upstream misfire, sensor issue, or fuel problem—is at fault.

What to do if you suspect a bad catalytic converter

Owners should take practical, timely steps to address potential converter problems and avoid further engine damage.

• Schedule a professional inspection promptly and avoid aggressive driving if you suspect a failure, to prevent overheating or additional damage.


• Check warranty coverage and recall status, as some converters are partially or fully covered under certain vehicle warranties or campaigns.


• Investigate related engine issues (misfires, faulty oxygen sensors, ignition problems) that can accelerate converter wear or produce false symptoms.

Acting quickly can help preserve engine health, reduce emissions, and minimize repair costs.

Summary

The catalytic converter plays a critical role in reducing vehicle emissions. Signs of a failing converter include a check engine light with codes like P0420/P0430, decreased power and fuel economy, rough idle or misfires, a rotten-egg odor, rattling noises, and potential emissions test failures. If you notice these signals, have the vehicle inspected by a qualified technician to confirm the cause and discuss replacement or repair options.

How does a car act when the catalytic converter is bad?

A car with a bad catalytic converter will exhibit symptoms like a Check Engine light, poor acceleration, and reduced engine performance because the converter is clogged and can't effectively clear exhaust gases. You may also notice a sulfur or "rotten egg" smell from the exhaust, dark exhaust fumes, and potentially hear a rattling noise from under the vehicle. 
Common symptoms of a bad catalytic converter

• Check Engine light: This is often the first sign, as the car's computer detects an issue with the emissions system. 
• Poor engine performance: The engine may feel sluggish, hesitate, or have a rough idle because of increased backpressure from a clogged converter. 
• Reduced acceleration: The vehicle may struggle to accelerate or reach highway speeds. 
• Sulfur or "rotten egg" smell: A failing converter may produce a smell like sulfur or rotten eggs when the engine is running. 
• Dark or visible exhaust fumes: The converter's job is to clean these gases, so dark or blue smoke indicates it's not functioning properly. 
• Rattling noise: The internal structure of the converter can break apart, causing a rattling sound from underneath the car. 
• Increased fuel consumption: The engine has to work harder to push exhaust through a blockage, leading to decreased fuel economy. 
• Excessive heat: A clogged converter can become extremely hot, potentially causing a fire hazard under the vehicle. 
• Failed emissions test: The car will likely fail an emissions inspection due to higher levels of pollutants in the exhaust. 
• Stalling: In severe cases, the engine can stall, especially at idle, because it can't "exhale" properly. 

What are the three most leading failures of a catalytic converter?

Catalytic Converter Failure: 3 Common Causes

• Unburned Fuel. Heat can be damaging to almost any engine component, so it's no surprise that it's one of the most common causes of catalytic converter failure.
• Coolant Leaks.
• Oil Consumption.

How do I know if my catalytic converter needs replacing?

Signs you may need a new catalytic converter include a check engine light, reduced engine performance, a rotten egg or sulfur smell, rattling noises, and poor fuel economy. A failing converter can also lead to a failed emissions test, difficulty accelerating, and dark exhaust smoke. 
This video explains the symptoms of a bad catalytic converter: 47sMaxTheCarGuyYouTube · Sep 8, 2024
Performance and driving issues

• Sluggish acceleration and poor performance: A clogged converter restricts exhaust flow, making the engine feel weak, sluggish, or slow to accelerate. 
• Reduced fuel economy: The engine works harder to push exhaust out, leading to decreased gas mileage. 
• Engine misfires or rough idling: Poor exhaust flow can cause the engine to run rough. 
• Difficulty starting: In severe cases, a completely blocked converter can prevent the engine from starting. 
• Overheating: A clogged converter can cause the area under the vehicle to become unusually hot. 

Sensory and warning signs

• Check engine light: This is a common indicator, though the specific code can relate to various issues like oxygen sensors. 
• Sulfur or rotten egg smell: This is a classic sign that the converter isn't converting gases properly. 
• Rattling noise: This can happen when the internal honeycomb material breaks apart and rattles around under the car. 
• Dark or unusual exhaust smoke: Black smoke is a sign of incomplete combustion, and a sulfur smell from the exhaust can indicate the converter has failed. 

Other indicators

• Failed emissions test: A malfunctioning converter will likely cause you to fail an emissions test. 
• Trouble with high RPMs: Acceleration may become difficult or limited past a certain engine speed. 

This video explains how to identify a bad catalytic converter: 53sScotty KilmerYouTube · Dec 22, 2015

How much will it cost to replace a catalytic converter?

Replacing a catalytic converter typically costs $900 to $3,500 or more, with the price heavily influenced by the vehicle's make and model, whether an OEM or aftermarket part is used, and the cost of labor. For many economy cars, the cost may be closer to the lower end, while luxury vehicles and trucks will likely be on the higher end due to more complex and expensive parts. 
Factors affecting the cost

• Vehicle type: The make, model, and year of your car are major factors. Luxury and heavy-duty vehicles often require more expensive, specialized converters. 
• Part type:
  • OEM (Original Equipment Manufacturer): These are original parts, which are often more expensive but can last longer. 
  • Aftermarket: These parts are less expensive, but their quality and lifespan can vary significantly. 
• Labor: Labor costs can range from approximately $75 to $150 per hour, depending on your location and the mechanic's shop rates. 
• Emissions standards: Some states have stricter emissions regulations, which may require more advanced and expensive catalytic converters. 
• Complexity of the job: Some vehicles have a catalytic converter built into the exhaust manifold, which can increase labor time and cost. 
• Additional parts: A replacement may also require replacing an oxygen sensor or other related components. 

Cost breakdown examples

• Economy Cars: Expect to pay between $500 and $1,200 for simpler models. 
• Mid-Size and Larger Vehicles: Costs are often between $800 and $1,800 due to more complex systems. 
• Trucks and SUVs: These can range from $1,000 to $2,500, with diesel models potentially costing more. 
• Luxury Vehicles: These can cost $2,000 to $4,000 or more due to the premium and advanced converters required. 

Tips for saving money

• Shop around: Get quotes from multiple repair shops to compare prices. 
• Check warranties: Your catalytic converter may be covered under a manufacturer's emissions warranty, which typically lasts for eight years or 80,000 miles. 
• Consider a universal converter: A universal converter can be a cheaper option, especially if you can find a shop willing to weld it into your exhaust system, potentially lowering costs significantly.