Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairMedium
A technician is performing a diagnosis on a vehicle with a complaint of a strong sulfur smell from the exhaust and a lack of power under acceleration. A scan tool shows a stored DTC P0420 (Catalyst System Efficiency Below Threshold, Bank 1). The upstream oxygen sensor (O2S11) switches rapidly, and the downstream oxygen sensor (O2S12) is also switching rapidly, mirroring the upstream sensor. What is the MOST likely cause of these symptoms?
- AA rich fuel condition.
- BA restricted exhaust system.
- CA faulty upstream oxygen sensor.
- DA failed catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: D. A failed catalytic converter.
When the downstream oxygen sensor (O2S12) mirrors the rapid switching of the upstream sensor (O2S11), it indicates that the catalytic converter is no longer storing and releasing oxygen effectively, meaning it has failed and is not converting exhaust gases as it should. The sulfur smell and lack of power further support this.
Why the other options are wrong
- A. A rich fuel condition could contribute to catalytic converter damage over time, but the direct symptom of the downstream O2 sensor mirroring the upstream indicates a converter failure, not just a rich condition in itself.
- B. A restricted exhaust system would cause a loss of power and potentially high exhaust back pressure, but it would not directly cause the downstream O2 sensor to mimic the upstream sensor's rapid switching.
- C. A faulty upstream oxygen sensor would likely cause fuel trim issues or a slow response DTC, not typically cause the downstream sensor to mirror it.
Catalytic Converter Efficiency
Catalytic converter efficiency is monitored by comparing the switching patterns of the upstream and downstream oxygen sensors. A healthy converter causes the downstream sensor to switch slowly and steadily, indicating effective oxygen storage and exhaust gas conversion.
- Upstream O2 sensor switches rapidly.
- Downstream O2 sensor should switch slowly/steadily (healthy converter).
- If downstream mirrors upstream, converter is failed (P0420).
Memory trick: Downstream follows upstream? Your cat's out of the game!