P0420
Category: Powertrain
Urgency:
Moderate Severity
P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Code P0420 means the oxygen sensor downstream of the catalytic converter is reporting that the converter is not effectively cleaning exhaust emissions.
Common Symptoms of P0420
- • Solid Check Engine Light illuminated
- • Slight reduction in engine power at high RPM
- • Rotten egg or sulfur smell from exhaust
- • Failed vehicle emissions / smog test
Root Causes for Code P0420
- • Degraded or failed catalytic converter substrate
- • Faulty downstream (Sensor 2) oxygen sensor
- • Exhaust leak near or before the catalytic converter
- • Engine running rich due to leaking fuel injectors or misfires
Step-by-Step DIY Diagnostic & Inspection Guide
1
Check for exhaust leaks before replacing the converter.
2
Compare upstream and downstream O2 sensor live voltage data with an OBD scanner.
3
Ensure no preceding misfire codes (P0300-P0308) are active.
4
Replace downstream O2 sensor or catalytic converter assembly.
Estimated Repair Costs
Replacement Parts:
$300 - $1,200
Shop Diagnostic / Labor:
$150 - $350
Total Estimated Cost:
$450 - $1,550
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