Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairMedium
A technician is performing an exhaust gas analysis on a vehicle with a P0420 (Catalyst System Efficiency Below Threshold, Bank 1) DTC. The readings at idle are: HC = 50 ppm, CO = 0.5%, O2 = 0.8%. After increasing engine RPM to 2500 for 2 minutes, the readings are: HC = 20 ppm, CO = 0.2%, O2 = 0.5%. What is the MOST likely conclusion?
- AThe catalytic converter is efficient.
- BThe engine is running excessively rich.
- CThere is an exhaust leak before the catalyst.
- DThe catalytic converter is failing.
Show answer & explanationAnswer & explanation
Correct answer: D. The catalytic converter is failing.
A P0420 DTC indicates the catalytic converter is not performing efficiently. While the numbers show some reduction in HC and CO (which a failing cat can still do partially), the fact that P0420 is set means it's below the required threshold for efficiency. A fully efficient catalyst would show much lower HC and CO, especially at elevated RPM, and typically lower O2 post-catalyst.
Why the other options are wrong
- A. If the cat were efficient, P0420 would not be set, and the HC/CO values would be significantly lower, often near zero.
- B. Excessively rich would show much higher CO and possibly HC, and likely O2 closer to 0% before the cat, which isn't definitively indicated.
- C. An exhaust leak before the catalyst would introduce unmetered air, potentially causing a false lean condition and affecting O2 readings, but not directly causing high HC/CO after the cat in this manner with a P0420.
Catalytic Converter Efficiency
Catalytic converter efficiency is measured by its ability to reduce harmful emissions (HC, CO, NOx) to less harmful gases. P0420 indicates it's below a set threshold.
- Monitored by upstream and downstream oxygen sensors.
- Reduces HC, CO, and NOx.
- P0420 indicates a failing converter.
Memory trick: A catalytic converter is a 'clean-up crew'; if it fails, emissions stay high.