EOBD/OBDII Error P0420 | Steps to Diagnose and Fix

The OBD-II error code P0420 indicates that the catalytic converter’s efficiency is below the required threshold for “bank 1,” which is the side of the engine with cylinder #1.

This can be caused by a faulty catalytic converter, issues with the oxygen sensors, exhaust leaks, or problems with the engine’s fuel system, spark plugs, or air filter.

To diagnose the issue, a mechanic should first check for other engine codes and then inspect the exhaust system, sensors, and spark plugs before concluding that the catalytic converter needs to be replaced.

What does “Bank 1” mean?

• Inline (straight) engines (e.g., 4-cylinder): There is only one exhaust bank, so this refers to the single catalytic converter.
• V-type engines (V6, V8, etc.): Bank 1 is the side of the engine that contains cylinder number one. The code P0430 is the equivalent code for Bank 2.

How the Code is Set

The catalytic converter’s efficiency is monitored by two oxygen ($\text{O}_2$) sensors:

1. Upstream $\text{O}_2$ Sensor (before the catalytic converter):

   • Measures the oxygen content entering the converter. Its reading should fluctuate rapidly.

2. Downstream $\text{O}_2$ Sensor (after the catalytic converter):

   • Measures the oxygen content leaving the converter. If the converter is working correctly, this reading should be relatively steady.

The P0420 code is set when the readings from the upstream and downstream $\text{O}_2$ sensors become too similar, indicating the catalytic converter is no longer effectively cleaning the exhaust gases.

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Common Causes of P0420

While a failing catalytic converter is the end result, it’s crucial to identify what caused the failure, as the converter rarely fails completely on its own.

Steps to Diagnose and Fix P0420

Do not immediately replace the catalytic converter. Follow these steps to correctly diagnose the root cause:

1. Check for Other Codes:

   • Scan for ALL Diagnostic Trouble Codes (DTCs).
   • Fix any Engine Misfire codes (P030X) or Fuel System codes first. If you don’t, the new converter will fail quickly.

2. Inspect the Exhaust System:

   • Visually check the entire exhaust system (especially the manifold, flex pipe, and gaskets) for leaks, cracks, or soot marks before the catalytic converter and $\text{O}_2$ sensors.
   • Inspect the wiring to both $\text{O}_2$ sensors for damage.

3. Test the $\text{O}_2$ Sensors (Requires a Scanner with Live Data):

   • Compare the voltage readings between the Upstream and Downstream $\text{O}_2$ sensors.
   • If the catalytic converter is healthy, the downstream sensor’s voltage should be relatively flat and stable.
   • If the $\text{O}_2$ sensor itself is faulty, replacing it is the cheaper solution.

4. Try a Catalytic Converter Cleaner:

   • In cases where the converter is only mildly fouled (often from excessive oil/fuel), pouring a specialized catalytic converter cleaning additive into the fuel tank and driving at highway speeds (higher RPM) may clear the code temporarily or permanently.

5. Replace the Catalytic Converter:

   • If all other components (sensors, wires, engine performance) are confirmed to be operating correctly, the catalytic converter itself is degraded and needs to be replaced. Ensure you use an OEM or high-quality aftermarket part to prevent the code from returning.