If your vehicle has a check engine light on, with a rough running engine, is slow to accelerate, or your car smells like rotten eggs, you will need to diagnose the problem with a scan tool. If the scan tool reads code P0420 or code P0430 “catalyst system efficiency below threshold bank 1 and/or bank 2”, this means the vehicle’s computer has detected an issue with the catalytic converter. Generally, if there is an issue with the catalytic converter, there is a larger issue elsewhere. It could be with leaking coolant, engine oil, or gas. Find out what it means and what to do if a check engine light appears for engine codes P0420 and P0430 with these steps.
How to Diagnose Catalytic Converters
What Is Code P0420 and Code P0430?
Codes P0420 or P0430 are trouble codes for the catalytic converters. The interior of the catalytic converter looks like a honeycomb, filter, or screen. The exhaust gases enter the filter where the catalytic converter burns the rest of the fuel. If the catalytic converter is broken, raw fuel will travel out the exhaust and can smell like rotten eggs.
Why Is a Check Engine Light on for Code P0420 and P0430?
An oxygen (O2) sensor is placed before the catalytic converter to monitor what the engine is doing. It keeps track of the air/fuel ratio, and what is burning and what isn’t. Another O2 sensor is placed after the catalytic converter and its job is to monitor the outgoing exhaust.
The check engine light will turn on for codes P0420 and/or P0430 when the O2 sensor behind the catalytic converter senses an issue.
Some catalytic converters have the exhaust manifold attached. Most vehicles with a V6 or V8 engine will have two catalytic converters. Four-cylinder engines will likely have one catalytic converter.
A vehicle will read a P0420 code for a catalytic converter and a P0430 code for a catalytic converter with an exhaust manifold.
What Are the Signs of a Bad Catalytic Converter?
Catalytic converters rarely fail on their own. Catalytic converter failure is usually a symptom of a bigger issue like oil leaking into the catalytic converter and causing it to heat up, or gas or coolant leaking. Sometimes the OBD-II system will read for additional codes like a misfire.
Common signs a catalytic converter is defective:
- Check engine light for codes P0420 and P0430: Catalyst System Efficiency Below Threshold Bank 1 or Bank 2
- Exhaust smells like rotten eggs
- Rough running engine
- Slower acceleration
- Engine misfire
Steps for Diagnosing Codes P0420 and P0430: Catalyst System Efficiency Below Threshold Bank 1 or Bank 2
Inspect the Air Intake System Parts
Inspect the air intake system for leaks, like the air intake tube and its clamps for looseness. Also check for intake leaks. If there is no accompanying fuel trim code, there are probably no fuel related issues.
Check the Exhaust System for Leaks
Check the flanges and the gaskets of parts like the cat back and the catalytic converter, and confirm these parts are connected tightly. This article further explains how to inspect the exhaust system.
How to Replace the Catalytic Converter Yourself
Mechanic’s Tip: Since catalytic converters rarely fail on their own, if the engine has another issue like a misfire, fluids can sneak in and burn up the converter if the original issue is not fixed.
Learn to replace your own catalytic converter and fix other problems like an engine misfire with 1A Auto’s how-to videos that have step-by-step instructions for many repairs on many makes and models.
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