Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A technician is diagnosing a vehicle with a persistent P0420 (Catalyst System Efficiency Below Threshold Bank 1) DTC. After verifying no exhaust leaks and proper oxygen sensor operation, the technician performs a 'warmed-up' catalyst efficiency test. The results show that the Bank 1 downstream oxygen sensor's voltage reading fluctuates significantly, mirroring the upstream oxygen sensor's fluctuations. Which of the following is the MOST likely cause?
- AA contaminated catalytic converter.
- BA faulty upstream oxygen sensor.
- CAn exhaust leak after the downstream oxygen sensor.
- DA restricted catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: A. A contaminated catalytic converter.
In a healthy catalytic converter, the downstream oxygen sensor should show a relatively stable voltage, indicating that the catalyst is efficiently storing and releasing oxygen. If the downstream sensor mirrors the upstream sensor, it means the catalyst is no longer performing its job of converting exhaust gases, which is a classic symptom of a contaminated or failed catalytic converter.
Why the other options are wrong
- B. A faulty upstream oxygen sensor would lead to incorrect fuel trim adjustments and potentially P0171/P0172 DTCs, but it wouldn't directly cause the downstream sensor to mirror it in a P0420 context.
- C. An exhaust leak after the downstream oxygen sensor would not affect the sensor's ability to mirror the upstream sensor; it might cause other issues but not directly a P0420.
- D. A restricted catalytic converter would cause symptoms like poor power and potentially misfires, but its efficiency monitoring (O2 sensor mirroring) might not be the primary indicator.
Catalytic Converter Efficiency Test
A diagnostic procedure that assesses the catalytic converter's ability to store and release oxygen by comparing the activity of the upstream and downstream oxygen sensors.
- Healthy cat: Downstream O2 sensor shows stable voltage.
- Failed cat: Downstream O2 sensor mirrors upstream O2 sensor.
- P0420/P0430 DTCs indicate inefficiency.
Memory trick: Downstream mirroring, cat's not clearing, emissions fearing.