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P0420 - Catalyst System Efficiency Below Threshold Bank 1

Possible causes
- Three way catalyst converter Bank 1
- Exhaust tube
- Intake air leaks
- Fuel injectors may be faulty
- Fuel injector leaks
- Spark plugs may be faulty
- Improper ignition timing
- Engine Control Module (ECM)
What does this mean? What does this mean?
Tech notes
As the code description implies the P0420 code means that the vehicle's control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.
What does this mean? What does this mean?
When is the code detected?
- Three way catalytic converter Bank 1 does not operate properly.
- Three way catalyst converter Bank 1 does not have enough oxygen storage capacity.
Possible symptoms
- Engine Light ON (or Service Engine Soon Warning Light)
P0420 Description

The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor).

A three way catalyst converter (Manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase.

When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst malfunction is diagnosed.

OBDII Code P0420 - Catalyst System Efficiency Below Threshold Bank 1 - AutoCodes.com
P0420 Information for specific makes
P0420 ACURA Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 AUDI Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 BMW Catalytic Converter Conversion  P0420 Image
P0420 BUICK Catalyst System Low Efficiency Bank 1
P0420 CADILLAC Catalyst System Low Efficiency Bank 1
P0420 CHEVROLET Catalyst System Low Efficiency Bank 1
P0420 CHRYSLER Catalyst System Efficiency Bank 1  P0420 Image
P0420 DODGE Catalyst System Efficiency Bank 1  P0420 Image
P0420 FORD Catalyst System Efficiency Below Threshold Bank 1  P0420 Video  P0420 Image
P0420 GMC Catalyst System Low Efficiency Bank 1
P0420 HONDA Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 HYUNDAI Warm Up Catalyst Efficiency Below Threshold Bank 1  P0420 Image
P0420 INFINITI Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 ISUZU Catalyst System Low Efficiency Bank 1  P0420 Image
P0420 JAGUAR Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 JEEP Catalyst System Efficiency Bank 1  P0420 Image
P0420 KIA Warm Up Catalyst Efficiency Below Threshold Bank 1  P0420 Image
P0420 LEXUS Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 MAZDA Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 MERCEDES-BENZ Catalyst System Efficiency Below Threshold Bank 1
P0420 MINI Catalytic Converter Conversion  P0420 Image
P0420 MITSUBISHI Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 NISSAN Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 PONTIAC Catalyst System Low Efficiency Bank 1
P0420 SATURN Catalyst System Low Efficiency Bank 1
P0420 SCION Catalyst System Efficiency Below Threshold Bank 1
P0420 SUBARU Catalyst System Efficiency Below Threshold  P0420 Image
P0420 TOYOTA Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
P0420 VOLKSWAGEN Catalyst System Efficiency Below Threshold Bank 1  P0420 Image
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