Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairMedium
A vehicle owner reports a significant drop in fuel economy and a noticeable loss of power, especially under acceleration. The 'Check Engine' light is illuminated, and a DTC P0420 (Catalyst System Efficiency Below Threshold, Bank 1) is stored. During diagnosis, the technician measures exhaust back pressure before and after the catalytic converter. The reading before the converter is significantly higher than after. What does this indicate?
- AA clogged or internally damaged catalytic converter.
- BA properly functioning catalytic converter.
- CA restricted exhaust pipe downstream of the converter.
- DA lean air-fuel mixture causing catalyst damage.
Show answer & explanationAnswer & explanation
Correct answer: A. A clogged or internally damaged catalytic converter.
A significant pressure difference across the catalytic converter (higher before, lower after) directly indicates a restriction within the converter itself. This restriction causes increased back pressure, leading to loss of power and reduced fuel economy, and is consistent with a P0420 code.
Why the other options are wrong
- B. A properly functioning catalytic converter would have minimal back pressure difference, and would not cause P0420, power loss, or poor fuel economy.
- C. A restriction downstream of the converter would cause high back pressure both before and after the converter, not a significant difference across it.
- D. While a lean mixture can damage a catalyst over time, the back pressure measurement directly indicates a physical restriction, not the cause of the catalyst damage.
Catalytic Converter Restriction
A restricted catalytic converter causes high exhaust back pressure before the converter, leading to power loss, reduced fuel economy, and potential P0420 DTC.
- Measured by exhaust back pressure test.
- High pressure before, low pressure after converter.
- Symptoms: power loss, poor fuel economy.
Memory trick: Converter's clogged: back pressure builds, power yields, and P0420 seals!