ASE A1 Engine RepairGeneral Engine DiagnosisEasy
A customer complains of poor engine performance, reduced fuel economy, and a strong sulfur smell from the exhaust. A scan tool reveals DTC P0420 (Catalyst System Efficiency Below Threshold Bank 1). Which of the following components is MOST likely the cause?
- AOxygen sensor (upstream)
- BCatalytic converter
- CMass Air Flow (MAF) sensor
- DEngine Coolant Temperature (ECT) sensor
Show answer & explanationAnswer & explanation
Correct answer: B. Catalytic converter
DTC P0420 specifically indicates that the catalytic converter's efficiency is below the acceptable threshold, meaning it is failing to properly convert exhaust gases, leading to the reported symptoms and sulfur smell.
Why the other options are wrong
- A. An upstream oxygen sensor issue might cause rich/lean conditions and performance problems, but P0420 directly points to the catalyst's efficiency, which is monitored by the downstream O2 sensor in relation to the upstream one.
- C. A faulty MAF sensor would cause various performance issues due to incorrect air measurement, but P0420 specifically targets catalyst efficiency, not air metering.
- D. A faulty ECT sensor affects fuel mixture and engine starting/running, but does not directly cause DTC P0420 or a sulfur smell.
DTC P0420
DTC P0420 indicates 'Catalyst System Efficiency Below Threshold (Bank 1)', meaning the catalytic converter on bank 1 is not effectively reducing emissions, often due to internal failure.
- Commonly causes reduced engine performance and fuel economy.
- Can produce a 'rotten egg' or sulfur smell from the exhaust.
- Usually requires replacement of the catalytic converter.
Memory trick: P0420 means your cat's not clean, time for a new scene.