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 324 (CHARGING SYSTEM)SELF-DIAGNOSTIC SYSTEMDIAGNOSTIC TROUBLE CODE DEFINITIONS

1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Trouble Code Definitions

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Trouble Code Definitions

WARNING: This page is about a different car, the 2003 Oldsmobile Aurora and 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.

DIAGNOSTIC TROUBLE CODE DEFINITIONS

DTC ⁽¹⁾

Description

Battery/Source Voltage Low

Charging System Volts Low

Charging System Volts High

PCM Detects An Improper Crank Request Input

Voltage Out Of Range

Low System Voltage

High System Voltage

Improper Voltage Level On Output Circuit That Controls Starter Relay

High Signal Voltage

Improper PWM Signal

P1633 ⁽²⁾

Improper Ignition 0 Input

P1634 ⁽³⁾

Improper Ignition 1 Input

⁽¹⁾	Codes listed in this table are only for testing covered in this article. For complete DTC listing, see BODY CONTROL MODULES - AURORA article.

⁽²⁾	See DTC P1633: IMPROPER IGNITION 0 INPUT under DIAGNOSTIC TESTS under SELF-DIAGNOSTICS article in ENGINE PERFORMANCE.

⁽³⁾	See DTC P1634: IMPROPER IGNITION 1 INPUT under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS article in ENGINE PERFORMANCE.

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.