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.
Diagnostic Procedures
1996 GMC Savana Special 6.5 YSECTION Diagnostic Procedures
- Perform On-Board Diagnostic (OBD) System Check. After performing OBD system check, go to next step.
- Check if any MAP sensor, HO2S, misfire, fuel trim, VSS or ECT sensor DTCs are set. If any other DTCs are set, go to applicable DTC test. If no other DTCs are set, go to next step.
- Check for the following conditions:
- Ensure TWC system is original equipment installed by manufacturer.
- Check TWC system for damage (i.e., dents, severe discoloration caused by excessive temperatures, holes).
- Ensure internal converter rattle (caused by damaged catalyst) is not present.
- Check exhaust system between TWC system and rear converter flange for leaks, damage, or loose or missing hardware.
- Ensure HO2S bank 1, sensor 3 on 4.3L; or HO2S bank 1, sensor 2 on 5.7L and 7.4L is secure, and pigtail and wiring is not damaged or contacting exhaust.
On all models, if a problem was found, go to step 9). If a problem was not found, go to next step.
- Check all VCM grounds. If a problem was found, go to step 9). If a problem was not found, go to next step.
- Check all sensor grounds. If a problem was found, go to step 9). If a problem was not found, go to next step.
- Check for intermittent open in catalyst monitored HO2S signal and ground circuits. If a problem was found, go to step 9). If a problem was not found, go to next step.
- Check for intermittent short to ground on catalyst monitored HO2S signal and ground circuits. If a problem was found, go to step 9). If a problem was not found, go to next step.
- Replace TWC system. Check for possible misfire (DTC P0300) or engine mechanical problem. Go to step 10).
- Repair problem as necessary. Go to next step.
- Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter DTC P0420. Operate vehicle within the conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool does not indicate that this test ran and passed, repeat step 2).
- Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTC(s) are displayed, go to applicable DTC test.
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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.