How to Check if Your Catalytic Converter Is Bad

You can check if your catalytic converter is failing by scanning for trouble codes like P0420/P0430, noting symptoms such as power loss, sulfur (“rotten egg”) smell, or rattling, and performing simple tests like temperature comparison, vacuum/backpressure checks, and downstream O2-sensor behavior. In practice, you’ll confirm suspicions by ruling out misfires, fuel-trim issues, and exhaust leaks first, because these often trigger false catalyst codes or destroy a good converter. Below, we explain what to look for, safe DIY diagnostics, when to stop driving, and what to expect from professional repair.

What a Catalytic Converter Does—and Why It Fails

The catalytic converter turns harmful exhaust gases (CO, HC, NOx) into less harmful ones using precious-metal catalysts. Converters typically last well past 100,000 miles, but they can fail from sustained misfires or rich running (unburned fuel overheats and melts the substrate), oil or coolant contamination, physical impact, internal breakage, overheating from heavy loads, or simple age. A stolen or damaged converter will cause immediate noise and emissions problems.

Common Symptoms You Can Spot

The following signs frequently accompany a failing or restricted catalytic converter. One symptom alone isn’t definitive, but several together strengthen the case.

• Check Engine Light with catalyst efficiency codes: P0420 (Bank 1) or P0430 (Bank 2); sometimes P0421/P0431 (warm-up catalyst) or P0422/P0432 (main catalyst).
• Noticeable lack of power, especially at higher RPM or on hills; engine may feel “choked.”
• Rotten egg (sulfur) smell from the exhaust, often with rich-running conditions.
• Rattling from the converter area at startup or acceleration (broken ceramic substrate).
• Excessive heat around the converter; in severe cases, the converter can glow red.
• Worse fuel economy with no other explanation.
• Failed emissions test (high HC/CO/NOx or catalyst monitor not ready).
• Very loud exhaust or a missing section of pipe—common after theft—plus visible cuts or damage.

If you’re experiencing two or more of these symptoms—especially a catalyst code plus power loss—proceed with targeted tests or seek a professional diagnosis to avoid misdiagnosis.

DIY Checks You Can Safely Perform

These steps can help you differentiate a bad converter from issues that mimic it. Work on a cool exhaust when possible, use proper safety gear, and never touch hot components.

1. Scan for OBD-II codes and data. Look for P0420/P0430 and related faults. Check freeze-frame data. View O2 sensor behavior: the upstream sensor should switch rapidly; the downstream sensor should be relatively steady. If the downstream mimics the upstream’s rapid switching, converter efficiency is likely low. If available, review Mode $06 catalyst monitor results.
2. Rule out root causes first. Fix misfires (P0300–P030X), rich/lean conditions (P0172/P0171), and exhaust leaks ahead of the converter—any of these can set false catalyst codes or ruin a good cat. Address O2/AF sensor heater codes (e.g., P0135) and intake or vacuum leaks.
3. Temperature comparison (IR thermometer). After a 10–15 minute drive with some load, measure near the inlet and outlet shells. A healthy converter’s outlet is typically somewhat hotter than the inlet (often 50–100°F/28–56°C under load). If the outlet is not hotter—or is cooler—the converter may be inactive. If the converter glows or shows extreme heat, stop driving.
4. Backpressure or vacuum test for restriction. Using a backpressure gauge in an upstream O2 sensor port, readings much above about 1.5 psi at idle or 3 psi at 2,500 rpm suggest restriction. Alternatively, a manifold vacuum gauge that steadily drops as rpm is held around 2,500 can also indicate exhaust restriction.
5. Tailpipe flow feel and road test. Weak exhaust flow plus power loss, especially if the engine revs poorly (struggling to exceed 2–3k rpm), points to a clogged converter.
6. Shake and listen (when cool). A metallic rattle from the converter can indicate a broken substrate that may soon clog the exhaust.
7. Visual check. Look for crushed or blued converter shells, missing heat shields, fresh soot tracks (leaks), or cut pipes indicating theft.

No single DIY test is perfect; combine results. If data is mixed or you lack tools, a professional can confirm with more precise methods before you spend on a converter.

Interpreting Trouble Codes

Codes help narrow the cause but don’t condemn the converter by themselves. Here are the most relevant ones and what they usually mean.

• P0420/P0430: Catalyst system efficiency below threshold (Bank 1/2).
• P0421/P0431: Warm-up catalyst efficiency below threshold (often sensitive to short-trip driving).
• P0422/P0432: Main catalyst efficiency below threshold.
• Related contributors: P0300–P030X (misfires), P0171/P0174 (lean), P0172/P0175 (rich), O2/AF sensor or heater codes (e.g., P0135, P0155), which can skew readings.

Address any misfire, mixture, or sensor heater issues first. A converter test after these fixes is far more conclusive and may save an unnecessary replacement.

When to Stop Driving

A severely restricted or overheating converter can damage the engine and raise fire risk. Park the vehicle and arrange a tow if you notice the following.

• Severe power loss; engine won’t rev past 2,000–3,000 rpm or stalls under load.
• Loud rattling from the converter, suggesting loose ceramic pieces that can clog the exhaust.
• Converter or nearby exhaust glowing red, strong sulfur odor, or burning smells.
• Engine overheating or smoke from underbody components.

Continuing to drive can turn a repair into a larger engine or exhaust replacement—and poses a safety hazard.

Professional Diagnostics and Repair Options

A qualified technician can verify converter health with bidirectional scan tools, live O2/AF sensor waveforms, Mode $06 results, smoke tests for leaks, in-line backpressure gauges, borescopes, and 5-gas analyzers. If replacement is needed, expect these considerations:

Technicians will confirm no upstream issues remain, select an OE or approved aftermarket converter, and ensure compliance with local emissions rules. After installation, they’ll clear codes, verify readiness monitors, and road-test to confirm proper operation. Costs vary widely by vehicle and whether one or multiple banks are affected.

Prevention—and What Not to Do

Good maintenance dramatically extends converter life and avoids repeat failures after replacement.

• Fix misfires, rich running, oil burning, or coolant leaks immediately; these overheat or contaminate the catalyst.
• Maintain healthy O2/AF sensors and thermostats; keep air intake and PCV systems in good order.
• Repair exhaust leaks ahead of the converter; false oxygen readings can trigger catalyst codes.
• Avoid sealants with silicone near the intake/exhaust and never use leaded fuel.
• Be skeptical of “pass emissions” additives; cleaners may help mild sulfur fouling but won’t fix a melted, cracked, or empty converter.
• Do not remove or hollow out the converter; it’s illegal in the U.S. and many regions and will fail inspections.

Preventive care is cheaper than a converter, protects the engine, and keeps your vehicle legal and clean-running.

Quick Checklist

Use this condensed sequence to triage converter concerns before committing to major repairs.

• Scan for codes and examine freeze-frame data.
• Fix misfires, fuel-trim issues, and exhaust leaks first.
• Compare upstream vs. downstream O2 sensor behavior.
• Check inlet/outlet temperatures after a drive.
• Measure backpressure or use a vacuum gauge for restriction.
• If results align or are unclear, get a professional confirmation.

If three or more indicators point to the converter (symptoms, codes, tests), it’s likely failing or restricted.

FAQs

Can a bad O2 sensor mimic a bad catalytic converter?

Yes. A lazy or faulty O2/AF sensor—especially downstream heater failures—can trigger P0420/P0430 or skew readings. Verify sensor operation and fix related codes before condemning the converter.

Will a catalytic converter cleaner fix it?

Cleaners may help if the issue is mild sulfur fouling after lots of short trips. They won’t repair melted, cracked, or broken substrates, nor will they overcome a severe restriction. Address root causes first.

How long should a catalytic converter last?

Often 10+ years or 100,000–150,000 miles, but lifespan depends on engine health and driving patterns. Frequent misfires or oil burning can kill a converter quickly.

Is replacement covered by warranty?

In the U.S., the federal emissions warranty covers the converter for 8 years/80,000 miles (whichever comes first) on most light-duty vehicles. Some manufacturers offer longer coverage; check your warranty booklet and recall/TSB history.

How do emissions rules affect replacement?

Aftermarket converters must meet federal EPA standards in most states. California, New York, Colorado, and Maine require CARB-approved units that are application-specific; using a non-CARB converter there can fail inspection. Diesels use DPF/SCR systems with different requirements. Always match the part to your VIN and retain documentation.

Summary

To check if your catalytic converter is bad: look for catalyst codes (P0420/P0430), verify symptoms like power loss or rattling, and perform temperature, O2-sensor, and backpressure/vacuum tests—after fixing misfires, mixture problems, and exhaust leaks. Stop driving if there’s severe restriction or overheating, and confirm with a professional before replacement. Choose compliant parts for your state, and prevent repeat failures by maintaining a healthy engine.

Can a scanner detect a bad catalytic converter?

If it’s not working correctly, it can lead to reduced fuel efficiency, poor engine performance, and of course, failing your emissions test. Luckily, with the help of a car scanner, you can get a pretty good idea of whether your catalytic converter is acting up without even needing to visit a mechanic right away.

Can catalytic converters be tested?

Yes, you can test a catalytic converter without removing it by using an infrared thermometer to check if the outlet is significantly hotter than the inlet, indicating it’s functioning correctly. Alternatively, a back pressure test, or using a scan tool to monitor O2 sensor readings, can also reveal if the converter is restricted or failing to convert exhaust gases properly. 
Method 1: Temperature Test (Infrared Thermometer)
This is a common and simple test to perform. 

1. Warm up the engine: Drive the car for about 15 minutes to bring the engine and catalytic converter to operating temperature. 
2. Measure inlet temperature: Point the infrared thermometer at the exhaust pipe just before the catalytic converter. 
3. Measure outlet temperature: Measure the temperature of the exhaust pipe just after the catalytic converter. 
4. Analyze the results:
   • Good: A functioning catalytic converter will show a significant temperature difference, with the outlet being much hotter (100-200°F difference is a good indicator) than the inlet. 
   • Bad: If the outlet temperature is the same as or lower than the inlet, the converter is likely not functioning correctly or is clogged. 

Method 2: Scan Tool Test
For vehicles with a compatible scan tool, you can use its live data feature. 

1. Connect the scan tool: Plug the scan tool into your vehicle’s OBD-II port. 
2. Monitor downstream O2 sensor: Look for the downstream (after the catalytic converter) oxygen sensor’s voltage. 
3. Analyze the results:
   • Good: The downstream O2 sensor voltage should be steady and relatively constant, hovering around 0.5V. 
   • Bad: If the voltage fluctuates wildly, it indicates the catalytic converter isn’t properly converting exhaust gases. 

Method 3: Back Pressure Test
This test checks for restriction within the exhaust system. 

1. Locate the upstream O2 sensor: Find the oxygen sensor before the catalytic converter. 
2. Remove the sensor: Carefully remove the sensor. 
3. Install a gauge: Attach a back pressure gauge using an appropriate adapter in place of the sensor. 
4. Run the engine: Start the engine and check the pressure reading at idle and around 2,000 RPM. 
5. Analyze the results: 
   • Good: Normal readings are low (e.g., under 1.5 psi at 2000 RPM). 
   • Bad: High pressure indicates a restriction, which could be a failing catalytic converter. 

Other indicators of a bad catalytic converter: Sluggish engine performance, Dark exhaust smoke, and A rattling sound from the exhaust when tapped with a mallet. 
Important Note: Some of these tests, like loosening the exhaust, may be illegal or impractical to perform in certain areas, so it’s important to be aware of local regulations.

How do you unclog your catalytic converter?

To unclog a catalytic converter, you can try several methods, including pouring a specialized catalytic converter cleaner into your fuel tank and driving the car for a short distance, or using an in-line spray cleaner after removing an oxygen sensor to gain access to the converter’s interior. Alternatively, a more involved process is to remove the converter and soak it overnight in a strong degreasing solution, then pressure wash it. If these methods don’t work or if the converter is severely damaged, replacement is the only option, as severely clogged converters are often melted and cannot be truly “unclogged”. 
Using fuel-tank additives

1. Purchase a catalytic converter cleaner: designed to be added to your fuel tank. 
2. Add the cleaner: to your fuel tank according to the product’s instructions. 
3. Drive your car: for a short distance or at higher speeds to allow the cleaner to work its way through the system and burn off deposits. 

Using a spray cleaner 

1. Locate the catalytic converter: and an oxygen sensor nearby.
2. Remove the oxygen sensor: by unscrewing it from its fitting.
3. Spray the cleaner: through the opening with the included straw.
4. Reinstall the oxygen sensor: and then drive the car to allow the cleaner to flush out the system.

Using a soak-and-pressure wash method

1. Remove the catalytic converter: from the vehicle. 
2. Submerge it: in a bucket of a strong degreasing solution overnight. 
3. Pressure wash the converter: to blast away the gunk, using the lowest pressure setting to avoid damage. 

Important Considerations

• Check for underlying issues: that may have caused the converter to fail. 
• For severe clogs, replacement is necessary . 
• Use an in-scope camera: to visualize the converter’s condition before and after cleaning. 
• Be cautious with extreme methods: like burning the converter, which can cause further damage or be dangerous. 

How do you diagnose a bad catalytic converter?

You can diagnose a bad catalytic converter by looking for a sulfur/rotten egg smell, reduced engine performance (sluggishness, poor acceleration), a check engine light, a rattling noise from the exhaust, and misfiring or jerking sensations. A technician can use a scan tool to check oxygen sensor data, measure the inlet and outlet temperatures with an infrared thermometer (outlet should be hotter), or check for excessive back pressure using a specialized gauge. 
Common Symptoms to Look For

• Rotten Egg Smell: The most common indicator is a smell of sulfur or “rotten eggs” from the exhaust. 
• Reduced Engine Performance: You might notice a lack of power, poor acceleration, or a generally sluggish feeling because the engine can’t “breathe” properly. 
• Check Engine Light: A failing catalytic converter can trigger the check engine light on your dashboard. 
• Engine Misfires: The increased heat from a clogged converter can cause fuel to ignite prematurely, leading to engine misfires. 
• Rattling Noises: If the internal honeycomb structure of the converter breaks apart, it can create a rattling sound from the exhaust system. 
• Poor Fuel Economy: A struggling converter forces the engine to work harder, leading to increased fuel consumption. 

Professional Diagnostic Tests

• Scan Tool Data: Opens in new tabA technician can use a scan tool to view live data from the vehicle’s oxygen sensors (O2 sensors). The second O2 sensor (post-catalytic converter) should show stable, consistent readings, while the first sensor (pre-catalytic converter) will fluctuate to maintain the air-fuel ratio. 
• Temperature Test: Opens in new tabA healthy catalytic converter should be hotter at its outlet than at its inlet. An infrared thermometer can be used to check this temperature difference; a smaller or negative difference indicates a potential problem. 
• Back Pressure Test: Opens in new tabA professional can remove an oxygen sensor and install a back pressure gauge to measure exhaust system restrictions. Normal pressure should be low (around 1.5 PSI at idle, and less than 3 PSI at higher RPMs). 
• Temporary Removal of Oxygen Sensor: Opens in new tabA simple test involves temporarily removing the upstream oxygen sensor. If the engine’s performance improves significantly, it points to a clogged catalytic converter as the likely issue. Note: In many locations, this is illegal due to emissions regulations.