OBD2 Code P0420: Catalytic Converter System Efficiency Below Threshold (Bank 1)
The P0420 code means Catalytic Converter System Efficiency Below Threshold (Bank 1). Estimated repair cost: $400 – $2500.
📋 Diagnostic Reference
| OBD2 Code | P0420 |
| Category | Powertrain |
| Severity | Check Soon |
| Repair Cost | $400 – $2500 |
| Can You Drive? | ⚠️ Limited driving only — go directly to repair shop |
🔍 Symptoms
- Check Engine Light on
- decreased fuel economy
- sulfur/rotten egg smell
- poor acceleration
- failed emissions test
⚠️ Most Likely Causes (in order of probability)
- Faulty catalytic converter
- oxygen sensor malfunction
- exhaust leak
- engine misfire causing unburned fuel
- contaminated converter
🛠️ Diagnostic Procedure
Step 1: Check for other stored codes first — P0420 is often a symptom not the root cause
Check for other stored codes first — P0420 is often a symptom not the root cause
Step 2: Inspect exhaust system for leaks before the catalytic converter
Inspect exhaust system for leaks before the catalytic converter
Step 3: Monitor O2 sensor voltage with live data — rear O2 should be steady not mirrorin...
Monitor O2 sensor voltage with live data — rear O2 should be steady not mirroring front O2
Step 4: Perform backpressure test on catalytic converter
Perform backpressure test on catalytic converter
Step 5: Check for engine misfire or rich condition that could damage the new converter.
Check for engine misfire or rich condition that could damage the new converter.
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