P0420

• Check engine light comes on
• Failing an exhaust emissions test
• Slight loss of power or rough idle
• Higher fuel consumption
• Often no noticeable difference while driving

• EGR valve clogged with carbon deposits
• Catalytic converter efficiency has dropped (P0420/P0430)
• Loose or faulty fuel cap (EVAP leak)
• Leak in the EVAP purge valve or hoses
• Ageing oxygen sensors

1. Read the code and identify which emission system (EGR, EVAP, catalytic converter) it points to.
2. Remove and inspect the EGR valve for carbon deposits — test that the valve opens and closes.
3. Check the fuel cap — a loose or damaged cap triggers EVAP codes.
4. Watch live oxygen sensor data: compare the signals before and after the catalytic converter.
5. Smoke-test the EVAP system for leaks.

1. Clean the EGR valve with carbon cleaner; replace it if cleaning doesn't help.
2. Tighten the fuel cap, or replace it with a genuine part if damaged.
3. Replace a faulty oxygen sensor (a long-life genuine part is recommended).
4. If a catalytic converter fault is confirmed (P0420/P0430), replace the converter.
5. Repair EVAP leaks (hoses, purge valve, canister) and confirm with a smoke test.
6. Clear the code and confirm the repair by watching emission values live.

• Kia: Warm Up Catalyst Efficiency Below Threshold Bank 1

• P041FAIR System Switching Valve A Circuit Low
• P0421Warm Up Catalyst Efficiency Below Threshold
• P0422Main Catalyst Efficiency Below Threshold
• P0423Heated Catalyst Efficiency Below Threshold
• P0424Heated Catalyst Temperature Below Threshold
• P0425Catalyst Temperature Sensor

This page is general information; an exact diagnosis needs measurements and a technician’s check.