ASE G1 Auto Maintenance and Light RepairElectrical/Electronic SystemsMedium
A technician is diagnosing a vehicle with a 'CHECK ENGINE' light illuminated. They connect a scan tool and retrieve Diagnostic Trouble Code (DTC) P0420, indicating 'Catalyst System Efficiency Below Threshold (Bank 1)'. Which of the following is the MOST appropriate initial diagnostic step?
- APerform a compression test on all cylinders.
- BReplace the catalytic converter immediately.
- CInspect the fuel injectors for proper spray pattern.
- DCheck for exhaust system leaks before and after the catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: D. Check for exhaust system leaks before and after the catalytic converter.
DTC P0420 indicates that the catalytic converter is not performing efficiently. However, before replacing an expensive component like a catalytic converter, it is crucial to rule out other factors that can cause this code. Exhaust leaks, particularly before or between the oxygen sensors, can allow unmetered air into the exhaust stream, skewing sensor readings and falsely triggering P0420.
Why the other options are wrong
- A. A compression test is for engine mechanical health and while engine problems can affect converter life, it's not the first diagnostic step for a P0420 code.
- B. Replacing the catalytic converter without further diagnosis can be costly and may not fix the underlying issue if an exhaust leak or other problem is present.
- C. Faulty fuel injectors could cause misfires or rich/lean conditions, which might damage a converter over time, but directly checking spray pattern isn't the most immediate diagnostic step for a P0420 code before checking for leaks.
DTC P0420 Diagnosis
DTC P0420 (Catalyst System Efficiency Below Threshold) indicates a catalytic converter issue, but exhaust leaks or sensor problems must be ruled out before replacing the converter itself.
- Indicates catalytic converter inefficiency.
- Exhaust leaks are a common false trigger.
- Oxygen sensor function must be verified.
Memory trick: P0420: Before you Replace, Check the Exhaust Space!