Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a persistent Check Engine Light (CEL) and a P0420 diagnostic trouble code (DTC). The scan tool data shows that the Bank 1 Sensor 2 oxygen sensor voltage fluctuates rapidly, mirroring the Bank 1 Sensor 1 oxygen sensor. What is the most likely cause of this condition?
- AA rich fuel condition.
- BA faulty catalytic converter.
- CA faulty Bank 1 Sensor 1 oxygen sensor.
- DAn exhaust leak before the catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty catalytic converter.
The Bank 1 Sensor 2 (downstream) oxygen sensor is responsible for monitoring the catalytic converter's efficiency. If it mirrors the upstream sensor, it indicates the converter is not storing oxygen, which is a sign of a faulty catalytic converter.
Why the other options are wrong
- A. A rich fuel condition would affect both sensors but wouldn't cause the downstream sensor to mirror the upstream sensor's rapid fluctuations if the converter were functioning correctly.
- C. A faulty upstream sensor would likely cause different DTCs and fuel trim issues, but not typically mirror the downstream sensor in this manner for a P0420.
- D. An exhaust leak before the converter would typically cause a lean condition detected by the upstream sensor and potentially different DTCs, not a P0420 with mirroring sensors.
Catalytic Converter Efficiency Monitoring
The vehicle's engine control unit (ECU) monitors catalytic converter efficiency by comparing the signals from the upstream (pre-catalyst) and downstream (post-catalyst) oxygen sensors.
- Upstream sensor signals fluctuate rapidly.
- Downstream sensor signals should be relatively stable and high (indicating oxygen storage) if the converter is efficient.
- If downstream sensor mirrors upstream, converter is inefficient.
Memory trick: Cat's too fast? Downstream's a mirror, not a storage tank!