Bad Catalytic Converter Symptoms: 6 Signs It’s Failing
The most common signs of a bad catalytic converter are a check engine light with code P0420 or P0430, a rotten-egg (sulfur) smell from the exhaust, sluggish acceleration or loss of power when it clogs, a rattle from underneath the car, worse fuel economy, and a failed emissions test. A converter usually fails because of something else — misfires, oil burning or a coolant leak — so that cause must be fixed too.
Updated 2026-11-11 · By CarWhere Research · Owner-complaint data: NHTSA
Short answer
- Check engine light, most often P0420 or P0430 (catalyst efficiency below threshold)
- Rotten-egg or sulfur smell from the exhaust
- Sluggish acceleration, loss of top speed or stalling (a clogged converter)
- Rattle from under the car, especially at start-up
- Failed emissions or smog inspection
- A very hot, glowing converter or heat smell after a misfire
Is it safe to drive? A steady P0420 with a normal-feeling car is usually safe to drive short-term while you get it diagnosed. A flashing check engine light, a glowing-hot converter, or severe power loss means stop: a clogged or overheating converter is a fire risk and can damage the engine. Check your emissions warranty before paying — the converter is federally covered for 8 years / 80,000 miles.
Bad catalytic converter symptoms at a glance
| Symptom | What you notice | How urgent | Warning light / codes |
|---|---|---|---|
| Check engine light | Steady light, often after highway driving | OK to drive — schedule a check | P0420 (bank 1), P0430 (bank 2) |
| Sulfur smell | Rotten-egg smell from the tailpipe | Drive gently — get it checked | Lean/rich codes possible |
| Loss of power | Car won’t accelerate, feels choked, may stall | Stop driving | Clogged converter — often no specific code |
| Rattle underneath | Loose rattling from the exhaust, like stones in a can | Drive gently — get it checked | Broken substrate |
| Failed emissions test | Readiness monitors incomplete or high emissions | OK to drive — schedule a check | P0420/P0430 |
| Overheating converter | Heat smell, glowing red underneath after a misfire | Stop driving | Flashing check engine light; P0300–P0308 |
How to confirm it
Checks you can do
- Read the codes. P0420/P0430 alone points at the converter or its oxygen sensors; misfire or fuel-trim codes must be fixed first.
- Note any rattle when tapping the converter with the engine cold.
- Check your vehicle’s emissions warranty (in the warranty booklet) and recall status before paying for a converter.
What a shop tests
- Compare upstream and downstream oxygen-sensor signals; a good converter keeps the downstream signal steady.
- Exhaust back-pressure or vacuum test for a clogged converter.
- Check for exhaust leaks and failed oxygen sensors that can falsely set P0420.
- Find the root cause: misfire, oil consumption, coolant entering the combustion chamber, rich fuel mixture.
What causes it
- Misfires that send unburned fuel into the converter and overheat it.
- Oil burning or coolant leaks (head gasket) that coat and poison the catalyst.
- Age and high mileage.
- Physical damage from road debris; theft (converter cut off — a loud exhaust).
The typical fix
- Fix the root cause first (misfire, oil burning, coolant leak), or a new converter can fail again.
- Replace the converter with a part that is legal for your state — California and some other states require CARB-approved converters.
- Replace a faulty oxygen sensor or repair an exhaust leak if those set the code instead.
Replace parts only after testing confirms the cause. Several symptoms here overlap with other faults; when in doubt, have the car inspected.
Real-world evidence: NHTSA owner complaints
Across the 194 models in our NHTSA complaint snapshot (395,134 complaints, fetched July 13 – September 29, 2026), 2,219 owner complaints mention the catalytic converter. These models have the most:
| Model | Complaints mentioning it | Model years with the most |
|---|---|---|
| Kia Soul | 775 | 2014, 2015, 2016 |
| Hyundai Tucson | 127 | 2016, 2017, 2019 |
| Chevrolet Equinox | 97 | 2010, 2011, 2012 |
| Ford Explorer | 79 | 2013, 2014, 2020 |
| Hyundai Veloster | 54 | 2013, 2015, 2016 |
| Honda Pilot | 48 | 2016, 2017, 2019 |
| Kia Sorento | 45 | 2014, 2016, 2017 |
| Dodge Journey | 44 | 2012, 2016, 2017 |
Counts, not failure rates: models that sell in larger numbers collect more complaints, and an owner’s complaint is unverified. We count complaints filed under the matching NHTSA component group whose narrative names the part or symptom, de-duplicated by NHTSA complaint number. Model links go to each model’s years-to-avoid page. Data date: complaints fetched July 13 – September 29, 2026.
Related codes, warning lights and repair costs
Frequently asked questions
Can I drive with a P0420 code?
Usually short-term if the car runs normally, but you may fail an emissions inspection and it can mask new problems. Diagnose it soon.
Is the catalytic converter under warranty?
Federal law requires an emissions warranty covering the catalytic converter and engine computer for 8 years or 80,000 miles on 1995-and-newer light-duty vehicles; some states and manufacturers go further.
Can a bad oxygen sensor make it look like the converter is bad?
Yes. A lazy downstream O2 sensor or an exhaust leak can set P0420 on a converter that is fine, so test before replacing.
Will fuel additives fix a catalytic converter?
Not a failed one. Additives won’t repair a melted or poisoned substrate; fixing the root cause and replacing the converter does.
If your scanner shows a catalyst code, read the P0420 code guide.
Related symptoms
Sources
- U.S. EPA, Questions and Answers: Emissions Warranties for 1995 and Newer Cars and Trucks (EPA-420-F-09-048), accessed 2026-11-11.
- eCFR, 40 CFR 86.1806-17 — Onboard diagnostics for light-duty vehicles, accessed 2026-11-11.
- U.S. EPA, Vehicle Emissions Inspection and Maintenance (I/M) programs, accessed 2026-11-11.
- Mazda owner’s manual (Mazda3), Check Engine Light, accessed 2026-11-11.
- NHTSA, Check for open safety recalls by VIN, accessed 2026-11-11.
- NHTSA Office of Defects Investigation, Vehicle owner complaints, via the public complaints API (api.nhtsa.gov/complaints/complaintsByVehicle), fetched July 13 – September 29, 2026.
General information written by CarWhere Research, not a diagnosis of your vehicle. Owner’s-manual excerpts are examples; follow the manual and factory service information for your own year, make and model, and check NHTSA for open recalls before paying for a repair. As of 2026-11-11.