P0421 2017 GMC Sierra Code - Catalyst 1 Low Efficiency

P0421 indicates that the ECM has detected low efficiency in the warm-up catalyst system for Bank 1, meaning the catalytic converter isn't functioning properly during cold starts.

Symptoms include a check engine light, decreased fuel efficiency, reduced engine performance, and in some cases, a noticeable sulfur-like odor from the exhaust.

Common causes include a failing catalytic converter, faulty oxygen sensors, exhaust leaks, engine misfires, or poor-quality fuel.

Prolonged driving with this code can lead to further damage, such as overheating of the catalytic converter or worsening engine performance.

Diagnosis typically involves checking for exhaust leaks, testing oxygen sensors, inspecting the catalytic converter for damage, and reviewing freeze frame data with a scan tool.

Not always; the issue might be resolved by repairing exhaust leaks, replacing oxygen sensors, or addressing underlying engine problems like misfires.

Repair costs can range from $200–$2,000, depending on whether the issue is with the oxygen sensors, exhaust system, or catalytic converter.

While unlikely, a dirty air filter can affect engine performance and contribute to inefficiencies that may trigger this code.

Ensure regular engine maintenance, use high-quality fuel, avoid prolonged idling, and address misfires or exhaust leaks promptly.

Clearing the code won't fix the underlying problem; the code will likely return unless the root cause is addressed.