A P0421 code on your 2023 Mercedes-Benz GLC indicates the ECM has detected that the warm-up catalytic converter on bank 1 is not reaching efficiency standards. This specifically tests catalyst performance during the warmup phase.
Warm-Up Catalyst Function
Many vehicles use a close-coupled warm-up catalyst positioned near the exhaust manifold. Its purpose is to light off (begin functioning) quickly during cold starts when emissions are highest. P0421 indicates this first-stage catalyst isn't performing properly during the warmup period.
GLC Engine Configuration
The 2023 GLC uses turbocharged four-cylinder engines. Mercedes places catalysts close to the turbo outlet for rapid warmup. The warm-up period is critical for emissions—80% of a trip's emissions can occur in the first few minutes.
Potential Causes
The warm-up catalyst substrate may be damaged or deteriorated. Frequent short trips that don't fully warm the catalyst can cause gradual damage. Oil consumption problems coat the catalyst with deposits. Misfires during cold operation damage catalyst material. Contaminated fuel can poison the catalyst.
Diagnostic Considerations
Use Mercedes XENTRY to monitor catalyst warmup characteristics. Compare upstream and downstream O2 sensor behavior during the warm-up test cycle. Verify no cold-start misfires are occurring. Check for oil consumption that could affect the catalyst. A 2023 model should be under warranty if the catalyst has failed.