Hummer H2. Manual - part 1382

Monitored Coolant Temperature - Range: 40-215° C (40-419° F)

This data is the clusters measurement of the engine coolant temperature as provided by the PCM on the 
class 2 serial data line. 

DIAGNOSTIC TROUBLE CODE (DTC) LIST 

Diagnostic Trouble Code (DTC) List 

DTC P1258 

Circuit Description 

The powertrain control module (PCM) uses the engine coolant temperature (ECT) sensor to monitor the engine 
for an over temperature condition. This condition occurs when the coolant temperature is above 132° C (270° 
F). When an over temperature condition is present, DTC P1258 will set. The PCM will disable 2 groups of 4 
cylinders by turning OFF the fuel injectors. By switching between the 2 groups of cylinders, the PCM is able to 
reduce the temperature of the coolant. 

Conditions for Running the DTC 

z

DTCS P0117, P0118, P1114, and P1115 are not active.  

z

The engine is running.  

Conditions for Setting the DTC 

The engine coolant temperature is above 132° C (270° F) for 10 seconds or more. 

Action Taken When the DTC Sets 

z

The PCM will illuminate the malfunction indicator Lamp (MIL) during the first trip in which the 
diagnostic test has been run and failed.  

z

The PCM will signal the IPC to turn ON the Service Engine Soon indicator.  

z

The PCM will alternately disable two groups of four cylinders by turning OFF the fuel injectors.  

z

The PCM will store conditions which were present when the DTC set as Freeze Frame and File Records 
data.  

Conditions for Clearing the MIL/DTC 

z

The PCM will turn the MIL OFF after 3 consecutive trips that the diagnostic has been run and passed.  

z

The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.  

z

The DTC can be cleared by using the scan tool Clear DTC Information function.  

DTC

Diagnostic Procedure

Module

P1258

DTC P1258

PCM

 

2004 Hummer H2 

2004 ENGINE Engine Cooling - Hummer H2

  

DTC P1258 

SYMPTOMS - ENGINE COOLING 

Visual/Physical Inspection 

z

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

z

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

z

Inspect the surge tank reservoir for the proper coolant level.  

Intermittent 

Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to Testing for 
Intermittent Conditions and Poor Connections in Wiring Systems. 

Symptom List 

Refer to a symptom diagnostic procedure from the following list in order to diagnose the symptom: 

z

Engine Overheated Indicator Always On  

z

Engine Coolant Temperature Indicator Always On  

z

Coolant Heater Inoperative  

z

Engine Overheating  

z

Loss of Coolant  

z

Thermostat Diagnosis  

z

Engine Fails To Reach Normal Operating Temperature  

z

Fan Clutch Diagnosis  

z

Low Engine Coolant Indicator Always On  

ENGINE OVERHEATED INDICATOR ALWAYS ON 

Step

Action

Yes

No

Connector End View Reference: Cooling System Connector End Views

1

Was the Diagnostic System Check for 
Engine Cooling performed?

Go to Step 2

Go to Diagnostic System Check 

- Engine Cooling

2

Check the engine cooling fan for proper 
operation. 
Is the engine cooling fan operative?

Go to Engine 

Overheating

Go to Symptoms - Engine 

Cooling

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

functions. Refer to Cooling System Description and Operation .

 

2004 Hummer H2 

2004 ENGINE Engine Cooling - Hummer H2

  

Engine Overheated Indicator Always On

ENGINE COOLANT TEMPERATURE INDICATOR ALWAYS ON 

Engine Coolant Temperature Indicator Always On 

Step

Action

Yes

No

Connector End View Reference:Engine Cooling Schematics 
DEFINITION: One of the following engine coolant temperature indicators is always On: Engine 
Coolant, Engine Coolant Hot/Idle Engine, Engine Hot-AC Off, Engine Overheated/Stop Engine.

1

Did you perform the Engine Cooling Diagnostic 
System Check?

Go to Step 2

Go To Diagnostic System 

Check - Engine Cooling

2

Start the engine. 
Does the Engine Coolant Temperature indicator 
illuminate?

Go to Step 3

Go to Testing for 

Intermittent Conditions and 

Poor Connections in Wiring 

Systems

3

1. Install a scan tool.  

2. With the scan tool, observe the engine 

coolant temperature parameter in the 
powertrain control module (PCM) data list. 

Does the scan tool indicate that the coolant 
temperature is within the temperature range 
shown on the temperature gauge?

Go to Engine 

Overheating

Go to Step 4

4

Replace the instrument panel cluster (IPC). Refer 
to Instrument Panel Cluster (IPC) 
Replacement in Instrument Panel, Gauges, and 
Console. 
Did you complete the replacement?

Go to Step 5

-

5

Operate the system in order to verify the repair. 
Did you correct the condition?

System OK

Go to Step 2

Step

Action

Yes

No

Connector End View Reference:Cooling System Connector End Views 
DEFINITION: One of the following engine coolant temperature indicators is always On: Engine 
Coolant, Engine Coolant Hot/Idle Engine, Engine Hot-AC Off, Engine Overheated/Stop Engine.

1

Did you perform the Engine Cooling Diagnostic 
System Check?

Go to Step 2

Go To Diagnostic System 

Check - Engine Cooling

2

Start the engine. 
Does the Engine Coolant Temperature indicator 
illuminate?

Go to Step 3

Go to Testing for 

Intermittent Conditions and 

Poor Connections in Wiring 

Systems

3

1. Install a scan tool.  

2. With the scan tool, observe the engine 

coolant temperature parameter in the 
powertrain control module (PCM) data list. 

 

2004 Hummer H2 

2004 ENGINE Engine Cooling - Hummer H2

  

LOW ENGINE COOLANT INDICATOR ALWAYS ON 

Low Engine Coolant Indicator Always On 

Does the scan tool indicate that the coolant 
temperature is within the temperature range 
shown on the temperature gauge?

Go to Engine 

Overheating

Go to Step 4

4

Replace the instrument panel cluster (IPC). Refer 
to Instrument Panel Cluster (IPC) 
Replacement in Instrument Panel, Gauges, and 
Console. 
Did you complete the replacement?

Go to Step 5

-

5

Operate the system in order to verify the repair. 
Did you correct the condition?

System OK

Go to Step 2

Step

Action

Yes

No

Connector End View Reference:Engine Cooling Schematics

1

Did you perform the Engine Cooling Diagnostic System 
Check?

Go to 

Step 2

Go to Diagnostic System 

Check - Engine Cooling

2

1. Verify that the engine coolant is at the proper level.  

2. Turn ON the ignition, with the engine OFF.  

Does the instrument panel cluster (IPC) display Low 
Engine Coolant?

Go to 

Step 3

Go to Testing for 

Intermittent Conditions 

and Poor Connections in 

Wiring Systems

3

1. Install a scan tool.  

2. With a scan tool, view the Coolant Level Switch data 

in the Engine Data 2 data list.  

Does the scan tool display Low?

Go to 

Step 4

Go to Step 7

4

1. Disconnect the coolant level sensor electrical 

connector.  

2. Connect a 3-amp fused jumper between the coolant 

level switch signal circuit of the coolant level sensor 
electrical connector and the ground circuit of the 
coolant level sensor electrical connector.  

3. With a scan tool, view the Coolant Level Switch data 

in the Engine Data 2 data list.  

Does the scan tool display OK?

Go to 

Step 8

Go to Step 5

5

1. Connect a 3-amp fused jumper between the coolant 

level switch signal circuit of the coolant level sensor 
electrical connector and a good ground.  

2. With a scan tool, view the Coolant Level Switch data 

 

2004 Hummer H2 

2004 ENGINE Engine Cooling - Hummer H2