Catalyst Efficiency Below Threshold (Bank 1)
P0420 indicates that the oxygen sensor downstream of the catalytic converter on Bank 1 is detecting that the converter is not storing oxygen efficiently, meaning it's not reducing emissions as designed. This is often due to a failing catalytic converter, but can also be caused by exhaust leaks, faulty oxygen sensors, or engine misfires. The code sets when the rear O2 sensor's signal mimics the front sensor's signal, indicating reduced catalyst activity.
01. Most Common Causes
- Failed catalytic converter
- Exhaust leak before or after the converter
- Faulty oxygen sensor (upstream or downstream)
- Engine misfire causing unburned fuel to damage the converter
02. Symptoms You Will Notice
- Check Engine Light on
- Failed emissions test
- Possible slight decrease in fuel economy
- Rotten egg smell from exhaust (if converter is severely clogged)
03. Step-by-Step Diagnosis
Step 1: Scan for codes and verify P0420 is present. Check for other codes like misfire or O2 sensor codes.
Step 2: Inspect exhaust system for leaks, damage, or loose connections. Repair any leaks found.
Step 3: Use a scan tool to monitor O2 sensor data. Compare upstream and downstream sensor readings. If downstream sensor mirrors upstream, converter is likely bad.
Step 4: Perform a catalyst efficiency test or use a propane enrichment test to confirm converter failure.
Step 5: If converter is confirmed bad, replace it. Also replace any faulty O2 sensors or repair underlying issues like misfires.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, but the vehicle will likely fail an emissions test and fuel economy may suffer. Prolonged driving with a severely clogged converter can cause overheating and damage other components.
How much does it cost to fix?▼
Cost varies widely: $200-$500 for O2 sensor replacement, $500-$2,500 for catalytic converter replacement depending on vehicle make/model and labor rates.