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 MANUALSOLDSMOBILE1998ACHIEVA 3.1 M, 4T40E/MN4REPAIR AND DIAGNOSISENGINE PERFORMANCETESTING & DIAGNOSISENGINE CONTROLS - TESTS W/CODES - 3.1LDIAGNOSTIC TROUBLE CODESDTC P0420 - TWC SYSTEM LOW EFFICIENCYDIAGNOSTIC PROCEDURES
1998 Oldsmobile Achieva 3.1 M, 4T40E/MN4
Diagnostic Procedures
1998 Oldsmobile Achieva 3.1 M, 4T40E/MN4SECTION Diagnostic Procedures
- Perform POWERTRAIN ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK , then go to next step.
- Diagnose any other DTCs before proceeding. If no other DTCs are set, go to next step.
- Using scan tool, clear DTCs. Ensure A/C is off. Start and warm engine to normal operating temperature. Operate vehicle within the conditions for setting this DTC. Test may need to be performed up to 6 times in order to pass or fail. If test has been performed or attempted for more than 6 times and DTC has not passed or failed this ignition cycle, test may be aborting. See DIAGNOSTIC AIDS. If scan tool indicates that DTC ran and passed in this ignition cycle, system is okay. If scan tool does not indicate that DTC ran and passed in this ignition cycle, go to next step.
- Ensure TWC is an original equipment part. Check TWC for dents or discoloration caused by excessive heat. Ensure no internal damaged catalyst rattle exists. If a problem is found, repair as necessary. After repairs, go to step 9). If no problem is found, go to next step.
- Check exhaust system for leaks, damage, loose or missing hardware. If a problem is found, repair as necessary. After repairs, go to step 7). If no problems are found, go to next step.
- Ensure HO2S 2 is securely installed and that harness is not damaged or contacting exhaust. If a problem is found, repair as necessary. After repairs, go to step 8). If no problems are found, go to step 9).
- Repair exhaust system. After repairs, go to step 10).
- Repair HO2S sensor circuit. After repairs, go to step 10).
- Replace TWC. Check for possible engine misfire DTC or engine mechanical problem. After repairs, go to next step.
- Using scan tool, clear DTCs. Ensure A/C is off. Start and warm engine to normal operating temperature. Operate vehicle within the conditions for setting this DTC. Test may need to be performed up to 6 times in order to pass or fail. If test has been performed or attempted for more than 6 times and DTC has not passed or failed this ignition cycle, test may be aborting. See DIAGNOSTIC AIDS. If scan tool indicates that DTC ran and passed in this ignition cycle, go to next step. If scan tool does not indicate that DTC ran and passed in this ignition cycle, repeat step 2).
- If additional DTCs are present, diagnose affected DTCs. If no additional DTCs are present, system okay.
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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.