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 4.3 W, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEM - TESTS W/CODES - DTC P1273-P1536DTC P1415DTC P1415: AIR SYSTEM BANK 1 (4.8L, 5.3L & 6.0L "C" & "K" SERIES)DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 4.3 W, Standard
Diagnostic Procedures
1999 GMC Forward Control 4.3 W, StandardSECTION Diagnostic Procedures
- Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS INTRODUCTION article. After performing OBD system check, go to next step.
- Install scan tool. Start and operate engine for greater than 2 minutes. Ensure engine is operating at normal operating temperature. Using scan tool, command AIR pump on. Observe Bank 1 Sensor 1 HO2S voltage as AIR pump is enabled. If HO2S voltage drops to less than 350 mV, see DIAGNOSTIC AIDS . If HO2S voltage does not drop to less than 350 mV, go to next step.
- Check AIR system hoses/pipes for proper connections or heat damage. Repair as necessary. After repairs, go to step 7. If no problems were found, go to next step.
- Disconnect hose from check valve at left exhaust manifold. Using scan tool, command AIR pump on. Observe if air flows from AIR valve. If air flows from valve, go to step 7. If air does not flow from valve, go to next step.
- Repair blockage or restrictions in AIR hose between check valve and each bank. After repairs, go to step 8.
- Repair as necessary. After repairs, go to step 8.
- Replace check valve. After replacing check valve, go to next step.
- Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter this DTC. 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 DTCs 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.