What are signs of a bad catalytic converter?

The most common signs are a check engine light (often with codes such as P0420 or P0430), noticeable loss of power and poorer acceleration, reduced fuel economy, a sulfurous “rotten egg” smell from the exhaust, a rattling or loud exhaust, and excessive heat near the converter. If you notice any of these, have your vehicle inspected promptly.

Key indicators to watch for

The following indicators cover performance, smell, noise, and emissions. Some signs may overlap with other engine problems, but together they point toward catalytic converter issues.

• Check engine light with emissions codes such as P0420 (Catalyst System Efficiency Below Threshold) or P0430 (Catalyst System Efficiency Below Threshold – Bank 2).


• Decreased power and sluggish acceleration, especially when climbing hills or overtaking, due to increased exhaust backpressure.


• Lower fuel economy compared with typical performance for your vehicle and driving style.


• Rotten-egg or sulfur-like odor coming from the exhaust or around the tailpipe, sometimes more noticeable after a hot engine.


• Rattling, ticking, or loud exhaust noises from beneath the vehicle, often when starting the engine or at idle, indicating a broken internal substrate.


• Excessive heat under the vehicle or a converter that becomes unusually hot to touch after the car has run, signaling possible blockage or overheating.


• Failure to pass a state or national emissions test or abnormally high tailpipe emissions.


• Visible damage, missing, or tampered catalytic converter, which can occur from theft or impact damage.

These signs can also result from other issues such as misfiring engines, faulty oxygen sensors, or exhaust leaks. A professional diagnostic is recommended to confirm a bad catalytic converter.

Diagnosing and fixing

When a shop suspects a catalytic converter problem, they typically proceed with a structured diagnostic workflow to confirm the fault and determine the right remedy.

• Scan the vehicle’s onboard computer with an OBD-II scanner to retrieve stored codes and live data from oxygen sensors and the catalytic monitor.


• Perform a visual inspection of the exhaust system for damage, corrosion, leaks, or signs of tampering.


• Check for backpressure using a backpressure test or by observing exhaust flow characteristics to identify a blockage.


• Run catalyst efficiency tests or use a gas analyzer to measure exhaust gas concentrations and verify catalyst performance.


• Address root causes: repair misfires, replace failing oxygen sensors or fuel-system issues, fix leaks, and replace the catalytic converter if necessary.


• Consider required replacement: catalytic converters are sold as OEM or aftermarket units; replacement cost varies by vehicle and part quality, plus labor.

Because replacement affects emissions compliance and can involve theft-prone parts with expensive metals (platinum, palladium, rhodium), it’s important to source parts from reputable suppliers and have the work performed by a qualified technician.

Summary

Bad catalytic converters present a mix of performance, smell, sound, and emissions signs. If you notice a sudden drop in power, a sulfur odor, or a check engine light with relevant codes, have your car examined promptly. Proper diagnosis is essential, as many signs overlap with other issues, and timely replacement can prevent further engine damage and ensure you stay compliant with emissions standards.

Should I replace my catalytic converter or get a new car?

You should replace the catalytic converter if its replacement cost is significantly less than the car's value and the rest of the vehicle is in good condition. If the cost of the converter is close to or exceeds the car's worth, or if the car has other major mechanical issues, getting a new car might be a better investment. 
Decide based on cost and condition

• Replace the converter if:
  • The cost of a new converter is low compared to the car's value. 
  • The vehicle is otherwise in good running order, with no major mechanical problems. 
  • You want to save money, as it is generally cheaper to maintain an older car than to buy a new one. 
  • The issues caused by the failing converter (like poor engine performance) are seriously affecting your driving experience. 
• Get a new car if:
  • The cost of the catalytic converter is almost as much as or more than the car is worth. 
  • The car is an old "rust bucket" with many miles, and is likely to have other expensive repairs in the near future. 
  • You want the peace of mind of a new vehicle warranty. 

Before making a final decision

• Get a quote: Ask a mechanic for a quote on the replacement to know the exact cost. 
• Check the underlying cause: Before replacing the converter, have a mechanic check the engine to ensure there isn't another issue (like a rich or lean air-fuel mixture) causing the converter to fail. If the engine is not running properly, the new converter could be damaged quickly. 
• Consider aftermarket parts: You may be able to get a cheaper aftermarket catalytic converter installed by a muffler shop, which can make replacement a more affordable option. 

What can be mistaken for a clogged catalytic converter?

A bad catalytic converter can be mistaken for other issues like engine misfires, poor acceleration, and reduced fuel efficiency because a clogged converter causes exhaust back pressure and engine performance problems. Other symptoms such as the "rotten egg" smell, a Check Engine light, or rattling noises can also point to different mechanical failures or issues within the exhaust system itself. 
Symptoms and mistaken diagnoses

• Poor acceleration and engine performance: A clogged converter restricts exhaust flow, leading to a noticeable lack of power and sluggishness that can mimic other engine problems. 
• Engine misfires: A bad converter can cause misfires by causing too much back pressure, leading the engine's management system to alter the fuel mixture. However, misfires can also be caused by spark plugs, fuel injectors, or other engine components, which can then damage the converter over time. 
• "Check Engine" light: A clogged converter will often trigger a Check Engine light, but this light can also be caused by many other issues in the engine or emissions systems, such as a loose gas cap or faulty oxygen sensor. 
• "Rotten egg" smell: A sulfurous or "rotten egg" odor is a classic sign of a catalytic converter problem, but a faulty oxygen sensor can also cause an unusual smell. 
• Rattling noise: A rattling sound coming from under the car can indicate a broken or loose internal honeycomb structure inside the converter. However, it could also be a loose exhaust pipe, heat shield, or something else rattling in the exhaust system. 
• Poor fuel economy: Restricted exhaust can make the engine work harder, leading to a significant drop in fuel efficiency that can be similar to a problem with the fuel system, sensors, or even tire pressure. 

This video explains the signs of a bad catalytic converter: 59s8020 AutomotiveYouTube · Apr 14, 2024
How to properly diagnose

• Use an OBD2 scanner: A scanner can read the trouble codes that trigger the Check Engine light, which can help narrow down the issue. 
• Check exhaust back pressure: A professional can use a gauge to measure the pressure in the exhaust system. Too much pressure indicates a blockage. 
• Perform a vacuum test: A vacuum gauge connected to the intake manifold can help identify exhaust restrictions by showing unusual vacuum readings. 
• Inspect oxygen sensors: Oxygen sensors can become faulty over time and provide incorrect information to the engine computer, so they should be tested for proper function. 

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 does a car act when the catalytic converter is gone?

Warning signs of a failing catalytic converter
Fuel consumption increases for no obvious reason. Strange rattles come from the engine. Your car experiences sluggish acceleration. The engine misfires.