Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSHYUNDAI2003XG350 BASEREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCETESTING & DIAGNOSISENGINE CONTROL SYSTEM DIAGNOSIS (2 OF 2)DIAGNOSTIC TESTSDTC P0421: WARM UP CATALYST EFFICIENCY BELOW THRESHOLD (BANK 1)GENERAL INFORMATIONDTC DESCRIPTION
2003 Hyundai XG350 Base
DTC Description
2003 Hyundai XG350 BaseSECTION DTC Description
If the frequency ratio of front and rear HO2S is higher than threshold value, the PCM determines that a fault exists and a DTC is stored.
RENDER: 1.0x
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.