What is a catalytic converter efficiency code?
How the Efficiency Code Works
The engine computer compares the signals from the upstream oxygen sensor (before the converter) and the downstream oxygen sensor (after the converter). A healthy converter stores and releases oxygen, causing the downstream sensor to switch slowly.
If the downstream sensor switches as rapidly as the upstream sensor, the computer interprets this as reduced converter efficiency and sets a P0420 (Bank 1) or P0430 (Bank 2) code.
Common Causes of Efficiency Codes
The most common cause is a failing catalytic converter. However, other issues can mimic a bad converter, such as a faulty oxygen sensor, exhaust leaks, engine misfires, or oil/coolant burning.
Diagnosis should rule out these other causes before replacing the converter, as converters are expensive.
- Faulty downstream oxygen sensor
- Exhaust leak before or after converter
- Engine misfire or rich condition
- Oil or coolant consumption contaminating the converter
- Aging or damaged catalytic converter
What to Do About It
If you get a P0420/P0430, have a mechanic diagnose the root cause. They may check for exhaust leaks, test oxygen sensors, and inspect for misfires.
If the converter is truly bad, replacement is the fix. Costs vary widely: aftermarket converters can be $200-$800, while OEM units can exceed $1,000, plus labor.
Common mistakes
- Replacing the catalytic converter without checking for other issues like bad oxygen sensors or exhaust leaks.
- Ignoring the code, which can lead to failed emissions tests and further damage.
- Using a cheap aftermarket converter that may not last or meet emissions standards.
