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 MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 284 (ENGINE CONTROLS - TROUBLE SHOOTING - NO CODES)SYMPTOMSRESTRICTED EXHAUSTTEST PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic
Test Procedure
1999 GMC Forward Control 5.7 R, AutomaticSECTION Test Procedure
WARNING: This page is about a different car, the 2003 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.
- Did you verify the customers complaint? If yes, go to next step.
- Did you review the exhaust symptoms diagnostic information and perform the necessary inspections? If yes, go to next step. If not, check for any exhaust leaks.
- Is the system equipped with dual exhaust? If system has dual exhaust, see Diagnostic Aids . If system does not have dual exhaust, go to next step.
- Remove the HO2S that is in front of the catalytic converter. See HEATED OXYGEN SENSOR under SENSORS in REMOVAL & INSTALLATION - CORVETTE article. Install the Exhaust Back Pressure Gauge in place of the HO2S sensor. Start the engine. Increase and monitor the engine speed at 2500 RPM. Observe the reading on the Exhaust Back Pressure Gauge. If the reading exceeds 2 psi (14 kPa) go to next step. If reading does not exceed 1.25 psi (9 kPa), go to step 8 .
- Turn the engine off and place the ignition in the lock position. Remove the Exhaust Back Pressure Gauge. Re-install the HO2S sensor. Remove the post-catalyst HO2S sensor. See HEATED OXYGEN SENSOR under SENSORS in REMOVAL & INSTALLATION - CORVETTE article. Install the Exhaust Back Pressure Gauge in place of the post HO2S sensor. Start the engine. Increase and monitor the engine speed at 2500 RPM. Observe the reading on the Exhaust Back Pressure Gauge. If the reading exceeds 2 psi (14 kPa) go to next step. If reading does not exceed 2 psi (14 kPa), go to step 7 .
- Inspect exhaust system for damage in the exhaust pipe, debris in the exhaust pipe, muffler or resonator internal failure, or 2 layer exhaust pipe separation. Did you find and correct the condition? If yes, go to step 8 .
- Replace appropriate catalytic converter. After repairs are complete, go to next step.
- Remove the Exhaust Back Pressure Gauge. Reinstall the applicable HO2S sensor. See HEATED OXYGEN SENSOR under SENSORS in REMOVAL & INSTALLATION - CORVETTE article. Clear any codes. Road test the vehicle in order to verify the repair. Did you correct the condition? If yes, system is okay. If no, go to step 2 .
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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.