A steady check engine light with P0420 on your 2018 GMC Yukon indicates the catalytic converter isn't processing exhaust gases efficiently. Before replacing expensive converters on this full-size SUV, proper diagnosis confirms actual failure versus sensor or exhaust issues.
Understanding P0420
P0420 triggers when the downstream oxygen sensor shows switching patterns too similar to the upstream sensor, indicating the converter isn't adequately processing exhaust. On the Yukon's V8, this code refers to Bank 1 converter.
Yukon V8 Considerations
The 2018 Yukon's 5.3L or 6.2L V8 uses multiple catalytic converters. P0420 indicates Bank 1 (driver's side on most configurations) converter efficiency is below threshold. Similar codes may affect Bank 2 independently.
Common Causes
True catalyst degradation from age, overheating from misfires, or contamination from oil burning. Oxygen sensor degradation falsely indicating converter problems. Exhaust leaks affecting sensor readings. AFM-related oil consumption contaminating converters.
Diagnosis Before Replacement
Given converter replacement costs of $1,000-$2,000+ on a full-size SUV, verify actual failure. Test oxygen sensors, check for exhaust leaks, and confirm no ongoing engine issues that would damage a new converter.