Catalyst Efficiency Below Threshold (Bank 1)
P0420 means the catalytic converter on Bank 1 is no longer cleaning the exhaust efficiently enough to satisfy the ECM. The most common root cause is a worn-out converter that can no longer store oxygen, though a slow downstream oxygen sensor can mimic the same fault. Severity is moderate: the car still runs, but it will fail an emissions test and the problem worsens over time.
01. Most Common Causes
- The ceramic substrate inside the converter has broken down or been poisoned by oil, coolant, or excess fuel, so it can no longer store oxygen or reduce pollutants; the ECM sees the rear sensor signal start to copy the front sensor.
- The downstream oxygen sensor (Bank 1, Sensor 2) has aged or become contaminated and now switches too slowly, so its trace resembles the upstream sensor and the ECM reports low catalyst efficiency even when the converter is healthy.
- An exhaust leak ahead of or near the converter is drawing in false air; that extra oxygen skews the rear sensor reading and makes the ECM calculate an incorrect conversion efficiency.
- Misfires or a chronically rich mixture have overheated and partially melted the substrate; unburned fuel ignites inside the converter and destroys the precious-metal coating.
02. Symptoms You Will Notice
- Check Engine Light on
- Failed emissions test
- Possible reduced fuel economy
- Possible rotten egg smell from exhaust
03. Step-by-Step Diagnosis
Step 1: Scan for codes and verify P0420 is present. Check freeze frame data for conditions.
Step 2: Inspect exhaust system for leaks, damage, or loose connections.
Step 3: Monitor oxygen sensor data with a scan tool. Compare front (Sensor 1) and rear (Sensor 2) O2 sensor readings. If rear sensor mirrors front sensor, catalyst is likely bad.
Step 4: Check for engine misfires or fuel trim issues that could damage the converter. Repair any underlying problems before replacing the converter.
04. Tools You Will Need
- OBD2 scan tool with live data to compare the upstream and downstream oxygen sensor signals across the catalyst
- Infrared temperature gun to measure the temperature delta between the converter inlet and outlet
- Digital multimeter to check the oxygen sensor heater circuits and signal voltage
1. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, but you may fail an emissions test and fuel economy may suffer. Prolonged driving with a bad converter can cause further damage.
How much does it cost to fix?▼
Diagnosis typically costs $50-$150. Replacing a catalytic converter can range from $500 to $2,500 or more, depending on the vehicle and parts.