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 LREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCETESTING & DIAGNOSISENGINE CONTROL SYSTEM DIAGNOSIS (2 OF 2)DIAGNOSTIC TESTSDTC P0421: WARM UP CATALYST EFFICIENCY BELOW THRESHOLD (BANK 1)INSPECTION/REPAIRCOMPONENT INSPECTION
2003 Hyundai XG350 L
Component Inspection
2003 Hyundai XG350 LSECTION Component Inspection
- Check Rear HO2S.
- Visually/physically inspect the rear HO2S for the following conditions:
- Ensure that the HO2S is securely installed. (Pigtail and wiring harness not making contact with the exhaust pipe)
- Check for corrosion on terminals
- Check for terminal tension (at the HO2S and at the PCM)
- Any damage
- Has a problem been found in any of the above areas?
YES
- Repair as necessary and go to "VERIFICATION OF VEHICLE REPAIR " procedure
NO
- Go to 2 "Check TWC" as below.
- Visually/physically inspect the rear HO2S for the following conditions:
- Check TWC.
- Visually/physically inspect the three-way catalyst (TWC) converter for the following damage:
- Severe discoloration caused by excessive temperature
- Dents and holes
- Internal rattle caused by a damaged catalyst
- Also, ensure that the TWC is a proper original equipment manufacturer part.
- Has a problem been found?
YES
- Substitute with a known - good TWC and check for proper operation.
If the problem is corrected, replace TWC and go to "VERIFICATION OF VEHICLE REPAIR " procedure.
NO
- Substitute with a known - good PCM and check for proper operation.
If the problem is corrected, replace PCM and go to "VERIFICATION OF VEHICLE REPAIR " procedure.
- Visually/physically inspect the three-way catalyst (TWC) converter for the following damage:
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.