Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a persistent P0420 (Catalyst System Efficiency Below Threshold Bank 1) DTC. The technician performs a visual inspection and finds no exhaust leaks. Next, the technician monitors the upstream and downstream oxygen sensor readings. Which of the following conditions would MOST strongly indicate a failing catalytic converter?
- AThe upstream O2 sensor shows a relatively steady voltage, while the downstream O2 sensor fluctuates rapidly.
- BThe upstream O2 sensor fluctuates rapidly, while the downstream O2 sensor shows a relatively steady voltage.
- CBoth upstream and downstream O2 sensor readings fluctuate rapidly and similarly.
- DBoth upstream and downstream O2 sensor readings show consistently low voltage.
Show answer & explanationAnswer & explanation
Correct answer: C. Both upstream and downstream O2 sensor readings fluctuate rapidly and similarly.
A healthy catalytic converter stores oxygen and smooths out the exhaust gas fluctuations, causing the downstream O2 sensor to show a relatively steady voltage. If both sensors fluctuate rapidly and similarly, it indicates the converter is not performing its job of storing oxygen and reducing emissions.
Why the other options are wrong
- A. This pattern would suggest a problem with the upstream sensor or an extremely lean condition, not a failing catalytic converter directly.
- B. This describes a healthy catalytic converter, where the upstream sensor tracks engine mixture changes and the downstream sensor shows a steady, filtered reading.
- D. Consistently low voltage on both sensors might indicate a severe lean condition or sensor malfunction, not specifically catalytic converter failure.
Catalytic Converter Efficiency Test (O2 Sensors)
Catalytic converter efficiency is determined by comparing the switching patterns of the upstream and downstream oxygen sensors.
- Healthy converter: Upstream fluctuates rapidly, downstream is steady.
- Failing converter: Both sensors fluctuate rapidly and similarly.
- P0420/P0430 DTCs indicate below-threshold efficiency.
Memory trick: Upstream dances, downstream sleeps; if both dance, catalyst weeps.