Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairEasy
A customer complains that the 'Check Engine' light on their vehicle illuminates, and a scan tool retrieves Diagnostic Trouble Code (DTC) P0420, 'Catalyst System Efficiency Below Threshold (Bank 1)'. Which of the following is the most likely cause?
- AA misfire in one of the engine cylinders.
- BA faulty upstream oxygen sensor.
- CA contaminated or failed catalytic converter.
- DAn exhaust leak before the catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: C. A contaminated or failed catalytic converter.
DTC P0420 specifically indicates that the catalytic converter is not efficiently storing oxygen, which is a direct sign of a failing or contaminated catalyst.
Why the other options are wrong
- A. A misfire in an engine cylinder would usually set misfire codes (P030X) and can damage the converter over time, but P0420 itself points to the converter's current inefficiency.
- B. A faulty upstream oxygen sensor would typically cause fuel trim issues and different DTCs, not directly P0420.
- D. An exhaust leak before the catalytic converter could affect oxygen sensor readings but usually triggers fuel trim codes or different catalyst codes, not P0420 directly related to efficiency.
DTC P0420
DTC P0420 indicates 'Catalyst System Efficiency Below Threshold (Bank 1)', meaning the catalytic converter is not functioning optimally to reduce emissions.
- Monitors post-catalyst O2 sensor activity vs. pre-catalyst.
- Indicates converter's inability to store oxygen.
- Commonly caused by internal catalyst failure.
Memory trick: P0420: The '420' means the CAT is 'NOT' working efficiently.