Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisHard
A technician is diagnosing a vehicle with an intermittent 'Check Engine' light and a stored DTC P0420 (Catalyst System Efficiency Below Threshold Bank 1). The technician monitors live data and observes the upstream oxygen sensor (O2S11) switching rapidly between 0.1V and 0.9V, while the downstream oxygen sensor (O2S12) is also switching, but more slowly, between 0.3V and 0.7V. What does this data indicate about the catalytic converter?
- AThe catalytic converter is failing to store oxygen effectively.
- BThe catalytic converter is completely plugged.
- CThe catalytic converter is functioning efficiently.
- DThe upstream oxygen sensor is faulty.
Show answer & explanationAnswer & explanation
Correct answer: A. The catalytic converter is failing to store oxygen effectively.
A healthy catalytic converter should 'clean up' the exhaust, causing the downstream O2 sensor to show a relatively steady voltage (typically around 0.6-0.7V) compared to the rapidly switching upstream sensor. When the downstream sensor starts switching, even if slowly, it indicates the converter is no longer effectively storing oxygen and is failing to perform its function, triggering a P0420 code.
Why the other options are wrong
- B. A completely plugged converter would cause severe engine performance issues and potentially different O2 sensor readings due to backpressure, not just switching downstream sensor activity.
- C. Efficient converters show a steady downstream O2 sensor voltage, not switching.
- D. The upstream O2 sensor switching rapidly is normal and indicates it is functioning correctly to monitor exhaust gas before the converter.
Catalytic Converter Efficiency
The ability of a catalytic converter to reduce harmful emissions, assessed by comparing the activity of upstream and downstream oxygen sensors.
- Upstream O2 sensor should switch rapidly (0.1V-0.9V).
- Downstream O2 sensor should be relatively steady (around 0.6-0.7V) for a good converter.
- If downstream O2 sensor starts switching like upstream, converter efficiency is low (P0420).
Memory trick: Upstream WAVES, Downstream STAYS for good cats.