P0421 – OBD-II DTC

OBD-II DTC P0421 – Catalyst System Efficiency Below Threshold (Bank 1)

DTC P0421 meaning

DTC P0421 indicates that the Engine Control Module (ECM) has detected that the catalytic converter’s efficiency is below the expected threshold for Bank 1. This means that the catalytic converter is not effectively converting harmful exhaust gases into less harmful emissions, which can lead to increased pollution and potential engine performance issues.

DTC P0421 symptoms

Common symptoms associated with DTC P0421 may include:

• Check Engine Light (CEL) illuminated
• Decreased fuel efficiency
• Rough engine idle
• Increased emissions
• Loss of power during acceleration

DTC P0421 causes

Several factors can trigger DTC P0421, including:

• Faulty catalytic converter
• Exhaust leaks before the catalytic converter
• Faulty oxygen sensors
• Engine misfires
• Fuel system issues (e.g., rich or lean conditions)
• Contaminated or damaged catalytic converter

DTC P0421 seriousness

DTC P0421 is considered a moderate to serious issue. While it may not cause immediate engine failure, it can lead to:

• Increased emissions, which can harm the environment
• Potential damage to the catalytic converter if not addressed
• Reduced engine performance and fuel efficiency
• Failing emissions tests, which can affect vehicle registration

How to diagnose DTC P0421

To diagnose DTC P0421, follow these steps:

1. Use an OBD-II scanner to confirm the DTC and check for any related codes.
2. Inspect the exhaust system for leaks, especially before the catalytic converter.
3. Check the operation of the upstream and downstream oxygen sensors.
4. Examine the catalytic converter for physical damage or contamination.
5. Perform a visual inspection of the fuel system for any issues.
6. Conduct a smoke test to identify any vacuum leaks.

How to fix DTC P0421 step-by-step instructions

To fix DTC P0421, follow these steps:

1. Repair any exhaust leaks found during diagnosis.
2. Replace faulty oxygen sensors if they are not functioning correctly.
3. If the catalytic converter is damaged or contaminated, replace it.
4. Address any engine misfires or fuel system issues that may be affecting performance.
5. Clear the DTC using an OBD-II scanner and take the vehicle for a test drive to ensure the issue is resolved.

How to erase DTC P0421

To erase DTC P0421, follow these steps:

1. Connect an OBD-II scanner to the vehicle’s diagnostic port.
2. Turn on the ignition without starting the engine.
3. Navigate to the “Erase Codes” or “Clear DTCs” option on the scanner.
4. Confirm the action to clear the codes.
5. Disconnect the scanner and start the engine to verify that the code does not return.

Recommended tools for OBD-II diagnostics

Explore our expert picks: The Best OBD2 Scanners for 2025

Easy-to-use Multimeter for reliable diagnostics: AstroAI Auto-ranging Multimeter

Some of the links on this page may be affiliate links, which means we may earn a small commission if you make a purchase. This helps us keep our DTC database up to date and provide valuable content for free. We only recommend products that we have tested and believe to be the best in their class.