How do you check for a bad catalytic converter?

A bad catalytic converter can trigger a check engine light, reduce power and fuel economy, or cause emissions failures. In many cases, you’ll start with symptoms, then confirm with diagnostic codes and live sensor data before deciding on replacement.

Signs that a catalytic converter may be failing

Before you dive into tests, here are the common signs that point toward catalytic converter trouble.

• Illuminated check engine light with catalytic efficiency codes such as P0420 or P0430 (on most modern vehicles).


• Noticeable power loss or reduced acceleration, especially at highway speeds.


• Declining fuel economy and/or poor engine performance.


• Rattling or banging noises from the exhaust, particularly on startup or when accelerating.


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


• Visible damage to the exhaust system or an overheating heat shield near the cat.


• Failing emissions test where the cat is suspected to be the culprit.

These symptoms can also be caused by other issues, such as faulty oxygen sensors or misfiring cylinders, so a thorough diagnostic is essential.

How engineers and mechanics diagnose catalytic converter problems

Use these steps to determine whether the catalytic converter is the likely cause, starting with simple checks and moving to more involved tests.

1. Scan for codes with an OBD-II reader. Look specifically for P0420/P0430 and note any related freeze-frame data, as well as the engine’s readiness monitors.


2. Inspect the exhaust system visually for cracks, leaks, or damaged heat shields that could affect catalyst performance or masking symptoms.


3. Examine oxygen sensor data. Compare pre-cat (upstream) and post-cat (downstream) sensor signals. The upstream sensor should swing widely as the engine runs; the downstream sensor should be more stable if the cat is doing its job.


4. Perform a temperature delta check if you have temperature probes or an infrared thermometer. With a warm engine, the cat inlet should be notably hotter than the outlet when the converter is working. A small or absent delta suggests a failing or clogged converter.


5. Consider a backpressure test. A blocked converter increases backpressure in the exhaust and can be measured with a manometer or specialized gauge. High backpressure often indicates a clogged catalytic converter.


6. Do a physical “tap test” only as a quick check. A rattling inside the ceramic substrate can indicate internal breakage, but this method is not definitive and should be followed up by professional diagnostics.


7. Cross-check with emissions testing or a lab analysis if available. In some shops, a specific catalytic converter efficiency test may be performed to confirm catalytic performance beyond sensor signals.

Because many symptoms overlap with other engine issues, a careful process that rules out misfires, sensor faults, and exhaust leaks is essential before concluding the cat is failing.

DIY versus professional evaluation

While some checks can be done at home, diagnosing a catalytic converter’s health with confidence usually requires professional tools and expertise.

• DIY checks you can safely perform include visual inspection, running an OBD-II scan, and monitoring live sensor data with a reader. Ensure the engine is cool before handling any exhaust components.


• Professional tests may include a controlled backpressure test, catalytic temperature delta measurements, or a formal emissions-catalyst efficiency test. A mechanic can verify whether replacement is necessary and recommend OEM or aftermarket options.

Note that a P0420/P0430 code can be caused by sensor faults or engine problems, not just a failed catalyst. A comprehensive diagnosis helps avoid unnecessary replacement costs.

What to do if you suspect a bad catalytic converter

If diagnostics point to a catalytic converter replacement, take these steps to protect yourself and your vehicle.

• Avoid driving long distances or high-load driving if the cat is suspected to be clogged, as it can lead to overheating or damage elsewhere in the exhaust system.


• Check your vehicle’s warranty and any applicable recalls or emissions warranty coverage; some converters are covered for a certain period or mileage.


• Obtain a professional diagnosis to confirm the problem and estimate replacement costs.


• Choose between OEM and aftermarket catalytic converters. OEM parts match exact factory specifications, while aftermarket options can offer cost savings with varying quality and warranties.


• Factor in typical costs: installation, labor, and the converter itself; prices vary by vehicle make/model and region. Expect a broader range for aftermarket parts and potential labor differences.


• Ensure proper installation to maintain exhaust flow and avoid triggering future sensor faults; a reputable shop should calibrate sensors and clear codes after replacement.

Replacing a catalytic converter is a common repair, but addressing any engine or sensor issues that contributed to failure is important for long-term reliability and to pass future emissions tests.

Summary

Diagnosing a bad catalytic converter starts with recognizing symptoms, confirming with diagnostic codes (notably P0420/P0430), and validating with live data and, if needed, backpressure or temperature tests. Because many factors can mimic catalytic failure, a systematic approach—often with a professional diagnosis—helps ensure accurate conclusions and appropriate replacement.

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. 

What two methods can be used to test a catalytic converter?

You can confirm using: OBD-II Scanner: Look for diagnostic trouble codes. Back Pressure Test: A mechanic can measure exhaust back pressure. Infrared Thermometer: Compare inlet and outlet temperatures of the catalytic converter (inlet should be cooler than the outlet if it's working).

How to unclog a catalytic converter?

To clean a catalytic converter, first inspect for physical damage or clogging. Use a fuel additive designed to break down carbon deposits inside the converter. Alternatively, remove the converter and soak it in a specialized cleaning solution. Avoid harsh chemicals that can damage the substrate.

How much does it cost to replace a catalytic converter?

The total cost to replace a catalytic converter typically ranges from $900 to $3,500, but can be as low as $500 for economy cars or higher for luxury and heavy-duty vehicles. This price includes both the parts, which can cost $200 to $2,500 or more depending on the vehicle's make and model, and labor, which is often billed at $70 to $130 per hour.
 
Cost breakdown

• Parts: The cost of the catalytic converter itself varies widely from approximately $200 to over $2,500. 
  • Economy cars: ~$500-$1,200 for parts and labor 
  • Luxury/heavy-duty vehicles: ~$2,000-$4,000 or more due to complexity and precious metal content 
  • Associated hardware: Costs for additional parts like gaskets, sensors, and clamps can add an extra $20 to $150. 
• Labor: Installation typically costs between $70 and $130 per hour, depending on the shop and your location. 
• Total estimated cost:
  • Economy vehicles: $500-$1,200 
  • Mid-size cars: $800-$1,800 
  • Trucks and SUVs: $1,000-$2,500 or higher 

Factors that influence the cost

• Vehicle make and model: Luxury vehicles and those with more complex exhaust systems require more expensive parts. 
• Part type: The price difference between an original equipment manufacturer (OEM) part and an aftermarket part can be significant. 
• Number of converters: Some vehicles have multiple catalytic converters, increasing the total cost. 
• Labor rate: Shop labor rates vary by region and the type of shop you use. 

How to save money

• Shop around for parts: Aftermarket converters can be a cheaper option than OEM parts. 
• Get multiple quotes: Get estimates from different mechanics to compare prices for both parts and labor. 
• Consider DIY installation: If you are comfortable working on cars, you may be able to save on labor costs by replacing the converter yourself, especially if you purchase a direct-fit part.