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 it's 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 preset target temperature before a calculated air flow is accumulated 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 24.14 km/h (15 mph) over the key cycle.
^ Mass Air Flow average reading more than 23 gm/sec.
^ 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.

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 Engine Cooling.
^ Cooling fans operation. Refer to Electric Cooling Fan Diagnosis (Functional Check). Component Tests and General Diagnostics

^ Check for high resistance in wiring related to the ECT sensor. Also, check for poor connections at ECT sensor and PCM. Refer to Testing for intermittent and Poor Connections in Diagrams.
Inspect the harness connectors of the ECT and PCM for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. Refer to Connector Repairs in Diagrams.

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.