P0128

Diagnostic Chart:

Schematic:

SYSTEM DESCRIPTION
The purpose of the cooling system is to maintain an efficient engine coolant temperature level during all engine operating conditions. This is accomplished by using a thermostat to regulate the flow of coolant from the engine to the radiator. The thermostat also allows the engine to warm up faster by not allowing coolant flow through the radiator until the engine has reached its operating temperature. An Engine Coolant Temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the Powertrain Control Module (PCM) for engine control and as an enabling criteria for various diagnostics.

The PCM monitors total accumulated air flow to determine if the engine run time is sufficient to reach engine operating temperature. If the coolant temperature does not increase normally or does not reach the regulating temperature of the thermostat, diagnostics that use engine coolant temperature as enabling criteria, may not run when expected. If engine coolant fails to reach a pre-set target temperature before a calculated air flow is accumulated DTC P0128 will set.

CONDITIONS FOR RUNNING THE DTC
^ No MAF, IAT, ECT, or VSS DTCs present.
^ Coolant temperature is more than 40°C (-40°F).
^ Air temperature is more than -7°C (20°F).
^ Engine running for more than 4 minutes.
^ VSS average more than 15 MPH over the key cycle.
^ Mass air flow average reading more than 23 gm/s.
^ Vehicle driven more than 3 miles.

CONDITIONS FOR SETTING THE DTC
The length of time for the engine coolant to reach a pre-set temperature is more than it should be.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM will illuminate the Malfunction Indicator Lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.
^ If the misfire is determined to be catalyst damaging, the PCM will flash the MIL.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.
^ The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using a scan tool.

DIAGNOSTIC AIDS
DTC P0128 will set if the diagnostic does not pass within 25 minutes after start-up. If DTC P0128 does not reset, inspect the following items:
^ Cooling system - Coolant level.
^ Thermostat operation - Refer to Thermostat Diagnosis in Cooling System.
^ Cooling fans operation - Refer to Electric Cooling Fan Diagnosis (Functional Test) or Electric Cooling Fan Diagnosis (Table #1) or Electric Cooling Fan Diagnosis (Table #2) or Electric Cooling Fan Diagnosis (Table #3). Component Tests and General Diagnostics Component Tests and General Diagnostics Component Tests and General Diagnostics Component Tests and General Diagnostics
^ Inspect for high resistance in wiring related to the ECT sensor. Also, inspect for poor connections at the ECT sensor and PCM. Refer to Testing for Intermittent and Poor Connections in Diagrams for a poor connection at the PCM.

IMPORTANT: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection-Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension.
^ Damaged harness-Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault.
^ PCM and engine grounds for clean and secure connections

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
4. This step verifies the engine coolant temperature sensor is not skewed by comparing observed values to the Temperature vs Resistance table. If the sensor is OK, the table will send you to Engine Cooling to diagnose slow engine warm-up.