Four signs your catalytic converter is failing

The four most common signs are: the check engine light with efficiency codes (often P0420/P0430), a rotten-egg sulfur smell from the exhaust, noticeable loss of power or poor acceleration (especially at higher RPM), and a metallic rattling noise from under the car at idle or startup. These symptoms point to a catalytic converter that’s clogged, damaged, or no longer cleaning exhaust effectively, and timely diagnosis can prevent further engine or emissions-system damage.

The four most common warning signs

Drivers and technicians typically spot catalytic converter trouble through a consistent set of symptoms. The following items outline what you’re most likely to notice and why each matters.

• Check engine light with efficiency codes: Diagnostic trouble codes P0420 (Bank 1) or P0430 (Bank 2) indicate “catalyst system efficiency below threshold,” flagged by the upstream and downstream oxygen sensors disagreeing about how well the converter is storing oxygen and converting pollutants.
• Rotten-egg (sulfur) smell from the exhaust: A failing catalyst may allow hydrogen sulfide (H₂S) to pass through instead of converting it, creating a distinct sulfur odor, especially after hard acceleration or when the engine runs rich.
• Loss of power or poor acceleration: A clogged or melted ceramic honeycomb restricts exhaust flow, causing sluggish performance, hesitation, or stalling at higher loads; the engine may feel like it “runs out of breath” at speed.
• Rattling noise from under the vehicle: Broken or loosened catalyst substrate can rattle inside the converter shell, often most noticeable at cold start, idle, or when lightly blipping the throttle.

While each symptom can have other causes, together they strongly suggest the catalyst is compromised and merits targeted testing before parts are replaced.

Why these symptoms happen

The catalytic converter uses precious metals to convert harmful gases (CO, HC, NOx) into less harmful ones (CO₂, H₂O, N₂). When the honeycomb substrate cracks, melts, or becomes contaminated (by oil, coolant, silicone, or lead), conversion efficiency drops. The engine control module compares oxygen sensor signals before and after the converter; when both sensors start to look similar, it sets an efficiency code and illuminates the check engine light.

Clogging raises exhaust backpressure, choking the engine and reducing power. If the substrate breaks apart, the pieces can vibrate inside the shell, creating a rattle. An overwhelmed or damaged catalyst may also allow sulfur compounds to pass through, producing the rotten-egg smell.

How to confirm the diagnosis

Because other issues can mimic these symptoms, a few straightforward checks can separate a failing catalyst from problems like misfires, exhaust leaks, or bad oxygen sensors.

1. Scan for codes and data: Look for P0420/P0430 and review live O₂ sensor readings; downstream O₂ should be more stable than upstream if the catalyst is working.
2. Inspect for exhaust leaks: Leaks ahead of the converter can skew O₂ readings and trigger false efficiency codes; repair leaks before condemning the cat.
3. Rule out engine faults: Fix misfires, vacuum leaks, rich/lean conditions, or coolant/oil consumption first—these can damage a good converter or mimic failure.
4. Check backpressure or vacuum: Excess backpressure (via a gauge at the O₂ port) or a steady drop in manifold vacuum at higher RPM points to a restricted cat.
5. Temperature comparison: With an IR thermometer, the outlet of a healthy, hot catalyst typically runs hotter than the inlet under load; equal or cooler outlet temps can indicate poor conversion (use caution and proper procedure).
6. Tap test for rattle: With the engine off and cool, a gentle tap on the converter may reveal loose substrate; never strike hard or when hot.

If tests point to the converter, replacing it only after underlying causes are addressed will help the new unit last.

What to do if it’s failing

Don’t ignore catalyst issues—continued driving with misfires or a clogged converter can overheat the unit (even glowing red) and risk damage to nearby components. Address root problems like misfires, oil consumption, or coolant leaks first. When replacing, use an emissions-compliant converter (CARB-certified where required, such as in California, New York, and select other states). Typical parts-and-labor costs range from roughly $300–$2,500+, depending on vehicle, emissions certification, and whether OEM parts are required; modern OEM units often exceed $1,000.

Preventing premature failure

Good maintenance habits significantly extend catalytic converter life. Consider the following practices to protect your emissions system.

• Fix misfires and rich/lean conditions promptly to prevent fuel and heat damage to the substrate.
• Repair oil or coolant consumption issues (e.g., valve seals, head gasket) that can poison the catalyst.
• Use the correct engine oil and avoid silicone-based sealants upstream of the exhaust; contaminants can coat the catalyst.
• Ensure there are no exhaust leaks before the converter and that O₂ sensors function properly.
• Keep up with scheduled maintenance (air filter, spark plugs, PCV system) to maintain proper combustion.

Proactive repairs and proper fluids keep the converter from being overloaded or contaminated, preserving both performance and emissions compliance.

Summary

Four clear signs of a failing catalytic converter are: a check engine light with P0420/P0430, a rotten-egg exhaust smell, noticeable loss of power, and a metallic rattle from the converter. Confirm with scans, leak checks, backpressure or vacuum tests, and temperature comparisons, and always resolve root engine issues before replacing the cat to ensure a lasting fix.

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

Symptoms of a bad catalytic converter include a rotted egg smell, a check engine light, rough idling, poor acceleration, a sluggish engine, and a rattling noise from the exhaust. Other indicators are reduced fuel economy, failed emissions tests, and sometimes even a lack of power or the engine running hotter than usual. 
Common Symptoms

• Rotten Egg Smell: A sulfur or rotten egg smell from the exhaust is a classic sign that the catalytic converter isn’t converting sulfur compounds into less harmful gases. 
• Check Engine Light: The car’s computer can detect issues with the catalytic converter, triggering the check engine light. 
• Reduced Engine Performance: A clogged catalytic converter creates backpressure, making it difficult for the engine to expel exhaust gases. This leads to: 
  • Poor Acceleration: The engine struggles to gain speed, making the vehicle feel sluggish. 
  • Loss of Power: The engine may feel significantly weaker than usual. 
  • Rough Idle/Misfires: The restricted exhaust can cause the engine to misfire or run unevenly, leading to a rough idle. 
• Rattling Noise: If the internal components of the catalytic converter have broken loose, you might hear a rattling sound coming from under the car. 
• Decreased Fuel Economy: With increased backpressure, the engine works harder, burning more fuel and leading to worse gas mileage. 
• Failed Emissions Test: Because the catalytic converter is responsible for converting harmful emissions, a faulty one will fail an emissions test. 

What to Do
If you notice these symptoms, it’s important to have your catalytic converter inspected by a qualified technician. Driving with a bad catalytic converter can lead to more severe issues, including complete engine failure.

How to test if a catalytic converter is bad?

To test a catalytic converter, use a thermal (infrared) thermometer to check if the outlet is about 100°F hotter than the inlet when the engine is at operating temperature. Other tests include using a vacuum gauge to check for a restriction or listening for a rattle by wiggling the exhaust pipe. A failed emissions test or a rotten egg smell are also strong indicators of a bad converter. 
1. The Temperature Test

1. Warm up the engine: to normal operating temperature. 
2. Measure the inlet temperature: of the catalytic converter using an infrared thermometer. 
3. Measure the outlet temperature . 
4. Compare the temperatures: If the outlet is significantly hotter than the inlet (by about 100°F), the converter is working properly. If the outlet temperature is lower or the same, it may be clogged or failing. 

2. The Vacuum Gauge Test 

1. Attach a vacuum gauge: to a manifold vacuum port.
2. Check the idle reading, which should be around 18-22 inches of vacuum.
3. Increase the engine speed: and watch the gauge.
4. Observe the reading: On a healthy engine, the vacuum will drop briefly and then rise higher than the idle reading. If the vacuum drops and stays low or continues to drop as you increase engine speed, the catalytic converter is likely clogged.

3. The Rattle Test

1. With the engine off, safely raise the vehicle and locate the catalytic converter. 
2. Wiggle the exhaust pipe: and listen for any rattling sounds coming from inside the converter. 
3. Listen for a loose rattle, which indicates the internal components of the converter have broken apart and are likely the cause of the problem. 

4. Symptom-Based Checks

• Failed Emissions Test: . Opens in new tabA malfunctioning catalytic converter will fail emissions tests because it can’t properly convert pollutants. 
• Rotten Egg Smell: . Opens in new tabA strong smell of sulfur or rotten eggs coming from the exhaust is a classic sign of a failing catalytic converter. 
• Reduced Power/Poor Acceleration: . Opens in new tabA clogged converter can restrict exhaust flow, leading to a significant loss of power and poor acceleration. 

How long should a catalytic convertor last?

10 years
What’s The Average Lifespan Of A Catalytic Converter? Catalytic Converters should last at least 10 years, but they don’t need to be replaced once the 10 years are up. It just needs to get changed whenever you start to see those problems occur and if it’s recommended by our service technicians.

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

The three most common reasons for catalytic converter failure are engine misfires and unburned fuel that overheat and melt the converter, contamination from oil or coolant, and physical damage from impacts, corrosion, or road debris. These issues reduce the converter’s efficiency and can cause a range of symptoms, including poor performance, a sulfur smell, and the illuminated check engine light. 
Here’s a breakdown of these failure types:

1. 1. Engine Misfires and Unburned Fuel 
   • Cause: Engine misfires or a rich fuel mixture allow unburned fuel to enter the exhaust system and reach the catalytic converter. 
   • Effect: This excess fuel ignites in the converter, causing the ceramic core to overheat and melt or crack, leading to a total failure. 
   • Related Issues: Faulty spark plugs, issues with the fuel system (injectors, float), or problems with the oxygen sensor can all contribute to this. 
2. 2. Contamination
   • Cause: Substances like engine oil, coolant (antifreeze), or contaminated fuel can enter the exhaust and coat the internal honeycomb of the catalytic converter. 
   • Effect: The coating blocks airflow and the catalyst’s surfaces, reducing efficiency and eventually “fouling” or “poisoning” the converter. 
   • Related Issues: A leaking engine can lead to excessive oil or coolant in the exhaust, while using poor-quality gasoline can introduce harmful sulfur and other contaminants. 
3. 3. Physical Damage
   • Cause: The converter’s ceramic substrate can be damaged by external factors. 
   • Effect: This damage can include cracks, corrosion from road salt, or structural damage from impacts with road debris, leading to reduced effectiveness. 
   • Related Issues: Metal fatigue from thermal shock (like driving through cold water when the engine is hot) or even just age can also contribute to physical breakdown.