ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A technician is diagnosing an engine that exhibits a strong, rotten egg smell from the exhaust and reduced engine power. A scan tool shows a pending P0420 code. Which of the following components is MOST likely failing?
- AMass Air Flow (MAF) sensor
- BEGR valve
- COxygen sensor
- DCatalytic converter
Show answer & explanationAnswer & explanation
Correct answer: D. Catalytic converter
A rotten egg smell (sulfur), reduced engine power, and a P0420 code (Catalyst System Efficiency Below Threshold) are all classic symptoms of a failing catalytic converter. The converter is failing to process exhaust gases effectively.
Why the other options are wrong
- A. A faulty MAF sensor can cause various drivability issues and lean/rich conditions, but not typically a rotten egg smell or a P0420 code directly related to catalyst efficiency.
- B. A faulty EGR valve typically causes rough idle, hesitation, or pinging, but not a rotten egg smell or P0420.
- C. A faulty oxygen sensor can contribute to a P0420 code, but it's usually a symptom of the converter's inefficiency, not the primary cause of the rotten egg smell and power loss unless it's causing an extreme rich condition.
Catalytic Converter Failure Symptoms
A catalytic converter converts harmful exhaust gases into less toxic pollutants. Failure leads to specific symptoms and diagnostic trouble codes.
- Rotten egg smell (sulfur) from exhaust.
- Reduced engine power and acceleration.
- Diagnostic Trouble Code (DTC) P0420 (Catalyst System Efficiency Below Threshold).
Memory trick: Rotten eggs and P0420 mean the converter is broken.