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 800 (ENGINE COOLING SYSTEM)DIAGNOSTIC INFORMATION AND PROCEDURESDTC P0480 OR P0481TEST DESCRIPTION

1999 GMC Forward Control 5.7 R, Automatic

Test Description

1999 GMC Forward Control 5.7 R, AutomaticSECTION Test Description

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

The numbers below refer to the step numbers on the diagnostic table.

2. Listen for an audible click when the cooling fan 1 relay operates. Command both the ON and OFF states. Repeat the commands as necessary.
3. Listen for an audible click when the cooling fan 2 and the cooling fan 3 relays operate. Command both the ON and OFF states. Repeat the commands as necessary.
4. Tests for voltage at the coil side of the cooling fan 1 relay. The cool fan 1 fuse supplies battery positive voltage to the coil side of the cooling fan 1 relay.
5. Verifies that the PCM is providing ground to the cooling fan 1 relay.
6. Tests for voltage at the coil side of the cooling fan 2 relay. The cool fan 2 fuse supplies battery positive voltage to the coil side of the cooling fan 2 and cooling fan 3 relays.
7. Verifies that the PCM is providing ground to the cooling fan 2 relay.
8. Verifies that the PCM is providing ground to the cooling fan 3 relay.
9. Tests if ground is constantly being applied to the cooling fan relays.
10. Test the control circuit of the cooling fan relays for a short to voltage or an open.
11. Test the control circuit of the cooling fan relays for a short to ground.
12. Test the battery positive voltage circuit for a short to ground.

Fig 1: DTC P0480 Or P0481 Diagnostic Procedure (Steps 1 - 12)

Courtesy of GENERAL MOTORS CORP.

RENDER: 1.0x

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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.