Chevrolet Silverado / GMC Sierra. Service manual — part 1607

P0480

1. Ignition OFF, disconnect the cooling fan low relay from the underhood fuse block.

2. Ignition ON, verify that a test lamp illuminates between the fan low relay coil voltage

supply circuit and ground.

If the test lamp does not illuminate, test the fan low relay coil voltage supply circuit
for and open/high resistance.

3. Connect a test lamp between the fan low relay coil supply voltage circuit and the fan low

relay coil control circuit. With a scan tool, command the fan relay 1 ON and OFF. Verify
that the test lamp turns ON and OFF with each command.

If the test lamp remains illuminated with each command, test the relay coil control
circuit for a short to ground. If the circuit tests normal, replace the control module.

If the test lamp does not illuminate, test the relay coil control circuit for and open or a
short to voltage. If the circuit tests normal, replace the control module.

4. If the circuits test normal, replace the fan low relay.

P0481

1. Ignition OFF, disconnect the cooling fan high relay from the underhood fuse block.

2. Ignition ON, verify that a test lamp illuminates between the fan high relay coil voltage

supply circuit and ground.

If the test lamp does not illuminate, test the fan high relay coil voltage supply circuit
for and open/high resistance.

3. Connect a test lamp between the fan high relay coil supply voltage circuit and the fan high

relay coil control circuit. With a scan tool, command the fan relays 2 and 3 ON and OFF.
Verify that the test lamp turns ON and OFF with each command.

If the test lamp remains illuminated with each command, remove the fan control relay
and retest. If the test lamp now turns ON and OFF with the fan control relay removed,
replace the fan control relay. If the test lamp still remains illuminated when
commanded ON and OFF, test the fan high relay coil control circuit for a short to
ground. If the circuit tests normal, replace the control module.

If the test lamp does not illuminate, test the fan high relay coil control circuit for and
open or a short to voltage. If the circuit tests normal, replace the control module.

4. If all circuits test normal, replace the fan high relay.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

Relay Replacement (Attached to Wire Harness) or Relay Replacement (Within an
Electrical Center)

Underhood Electrical Center or Junction Block Replacement

Control Module References for ECM replacement, setup, and programming

DTC P1258

Diagnostic Instructions

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Review Strategy Based Diagnosis for an overview of the diagnostic approach.

Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC P1258

Engine Coolant Over temperature - Protection Mode Active

Circuit/System Description

The engine control module (ECM) uses the engine coolant temperature (ECT) sensor to monitor
the engine for an over-termperature condition. This condition occurs when the coolant
temperature is above a calibrated value for a calibrated length of time. The ECM will disable half
of the cylinders by turning OFF the fuel injectors. By disabling half of cylinders, the ECM is able
to reduce the temperature of the coolant.

Conditions for Running the DTC

The engine is running.

DTC P0117 or P0118 are not set.

Conditions for Setting the DTC

Coolant temperature more than 132°C (270°F) for greater than 10 seconds.

Action Taken When the DTC Sets

DTC P1258 is a Type A DTC.

The engine will operate in the Overheated Engine Protection Operating Mode.

The engine coolant temperature indicator lamp, if equipped, will illuminate.

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

The driver information center (DIC), if equipped, may display a message.

Conditions for Clearing the DTC

DTC P1258 is a Type A DTC.

Reference Information

Description and Operation

Cooling System Description and Operation

Instrument Cluster Description and Operation

Indicator/Warning Message Description and Operation

Audible Warnings Description and Operation

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

1. Observe the engine coolant level. The engine coolant level should be within operating

range. Refer to Capacities - Approximate Fluid, and Engine Coolant in the Owner's Manual.

2. Ensure that the vehicle has the correct engine coolant, with correct concentration, and is not

old, contaminated or contains additives. Refer to Recommended Fluids and Lubricants in
the Owner's Manual.

3. Inspect the cooling system for the following:

Leaks

Kinked or pinched hoses, especially at the radiator

Loose, missing, or damaged radiator air seals or deflectors

The radiator and A/C condenser for any air flow obstructions or bent fins-Refer to
Symptoms - Engine Cooling.

IMPORTANT: If DTC P1482 is set, diagnose that DTC first.

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

4. Command the cooling fan speed from 0% to 100% with a scan tool.

If the fan speed does not transition with the commanded states, refer to Cooling Fan
Inoperative.

Circuit/System Testing

1. Test the thermostat for correct operation. Refer to Thermostat Diagnosis.

2. Test the engine cooling system for overheating. Refer to Engine Overheating.

3. Inspect the water pump and coolant flow for correct operation. Refer to Water Pump

Replacement (LU3) or Water Pump Replacement (LY6, L76 and L92) or Water
Pump Replacement (LH6, LY2, LY5, and LMG) or Water Pump Replacement
(LMM).

4. Inspect the engine for worn/leaking/cracked cylinder heads and engine block. Refer to

Coolant in Combustion Chamber and Coolant in Engine Oil .

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

SYMPTOMS - ENGINE COOLING

Cooling System Description and Operation

Cooling Fan Description and Operation (w/o LFA)

Visual/Physical Inspection

Inspect for aftermarket devices which could affect the operation of the Cooling System.
Refer to Checking Aftermarket Accessories .

Inspect the easily accessible or visible system components for obvious damage or conditions
which could cause the symptom.

Intermittent

Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to

IMPORTANT: A small delay occurs before the ECM changes the cooling fan

speed.

IMPORTANT: Review the system operation in order to familiarize yourself with

the system functions. Refer to the following:

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado