Buick Regal: 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 Descriptors

DTC P1258

Engine Coolant Overtemperature - Protection Mode Active

Circuit/System Description

The engine control module (ECM) uses the engine coolant temperature sensor to monitor the engine for an over-temperature condition. This condition occurs when the coolant temperature is above a calibrated value. The ECM will alternately disable 2 groups of cylinders by turning OFF the fuel injectors. By switching between the 2 groups of cylinders, the ECM is able to reduce the temperature of the coolant.

Conditions for Running the DTC

• The engine is operating for greater than 30 seconds.
• DTC P1258 runs continuously once the conditions above have been met.

Conditions for Setting the DTC

The ECM detects that engine coolant temperature is warmer than 131ºC (268ºF) for greater than 2 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 IP will illuminate the coolant temperature indicator lamp and the driver information center (DIC), if equipped, may display a message.
• If the protection mode is active and an ECT sensor DTC sets, the protection mode will remain active at the start of the next drive cycle until the ECT sensor DTC runs and passes.

Conditions for Clearing the MIL/DTC

DTC P1258 is a Type A DTC.

Reference Information

Schematic Reference

Engine Controls Schematics (LGX)

Connector End View Reference

Component Connector End View Index

Description and Operation

• Cooling Fan Description and Operation
• Instrument Cluster Description and Operation
• Indicator/Warning Message Description and Operation

Electrical Information Reference

• Circuit Testing
• Connector Repairs
• Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

K20 Engine Control Module: Scan Tool Information for scan tool information

Circuit/System Verification

1. Ignition ON.

2. Verify DTC P0480, P0481, P0691, P0692, P0693, or P0694 is not set.

• If a DTC is set

Refer to DTC P0480, P0481, P0691, P0692, P0693, or P0694 (LTG) or DTC P0480, P0481, P0691, P0692, P0693, or P0694 (LGX).

• Go to next step: If no DTCs is set

NOTE: A short delay occurs before the ECM changes the cooling fan speed.

3. Verify the cooling fan operates at each available speed as commanded with a scan tool.

• Cooling fan does not operate

Refer to Cooling Fan Inoperative (LGX) or Cooling Fan Inoperative (LTG).

• Go to next step: Cooling fan operates

4. Verify that the engine does not overheat.

• Engine overheats

Refer to Engine Overheating.

• Go to next step: Engine does not overheat

5. Operate the vehicle within the Conditions for Running the DTC.

• If the DTC sets

Refer to Symptoms - Engine Cooling.

• Go to next step: If the DTC does not set

6. All OK.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

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