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Reasons a Catalytic Converter Can Fail or Work Below Efficiency
The check engine light is your car’s way of warning you about potential problems. One common cause is a faulty or underperforming catalytic converter. This critical component reduces harmful emissions by converting carbon monoxide, hydrocarbons, and nitrogen oxides into less harmful compounds. When it fails or operates below efficiency, your car’s performance, fuel economy, and emissions are affected — and the check engine light often comes on.
What a Catalytic Converter Does
The catalytic converter is part of your vehicle’s exhaust system. Its job is to process harmful exhaust gases into safer compounds before they exit the tailpipe. A properly functioning converter helps your car maintain fuel efficiency, run smoothly, and pass smog and emissions tests.
How a Catalytic Converter Triggers the Check Engine Light
Modern cars have oxygen sensors located before and after the catalytic converter. These sensors measure how effectively the converter processes exhaust gases. When the converter is working below efficiency, the sensors detect an imbalance, and the engine control unit (ECU) triggers the check engine light, often storing codes like P0420 or P0430, which indicate “Catalyst System Efficiency Below Threshold.”
Why a Catalytic Converter Can Work Below Efficiency
- Faulty oxygen sensors: Incorrect air-fuel mixture readings can prevent the converter from processing gases efficiently.
- Engine misfires: Worn spark plugs, ignition coils, or other issues can send unburned fuel into the exhaust, overheating or contaminating the converter.
- Clogging or carbon buildup: Over time, carbon deposits reduce airflow through the converter, lowering efficiency.
- Oil or coolant contamination: Leaks that reach the converter can coat its internal substrate, reducing its ability to convert gases.
- Age and wear: Like all components, catalytic converters degrade over time, losing some efficiency even if they appear intact.
Signs of a Below-Efficiency Converter
- Reduced acceleration or sluggish engine response
- Decreased fuel economy
- Rattling noises from the converter
- Sulfur or “rotten egg” smell from the exhaust
- Engine misfires or rough idling
Repair and Replacement Considerations
Catalytic converter replacement is costly — often hundreds to thousands of dollars. Sometimes addressing upstream issues, like oxygen sensors or spark plugs, can restore partial efficiency, but a damaged converter usually requires replacement. Driving with a below-efficiency converter can also harm other components, increasing repair costs over time.
Final Thoughts
A failing catalytic converter or one operating below efficiency is a common reason for the check engine light. Failing oxygen sensors often contribute to this problem by causing poor combustion and excess fuel in the exhaust. If you notice poor performance, lower fuel economy, or unusual exhaust odors, it’s important to act fast.
If repairs are too costly, selling may be the better option. At Los Angeles Cash for Cars, we buy vehicles in any condition — even with a check engine light or catalytic converter issues. Serving Venice, Santa Monica, Marina del Rey, Culver City, and Beverly Hills, we pay top cash for cars, trucks, SUVs, and classics.
Call (310) 893-0162 today for a fast cash offer and same-day pickup.
Drew Marks
Has been buying, selling, reconditioning and restoring cars since 2008. He is the owner of Los Angeles Cash for Cars, a trusted car buyer serving the greater Los Angeles area. With years of hands-on experience in used cars, classic car restoration, and automotive valuation, he’s passionate about educating the public on how to sell a car for cash or buy a reliable used vehicle with confidence.