P0128

CIRCUIT DESCRIPTION
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 some diagnostics.

The air flow coming into the engine is accumulated and used to determine if the engine has been driven within conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach 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 DTC P0128 will set.

This DTC will only run once per ignition cycle within the enabling conditions.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0100, P0101, P0103, P0110, P0111, P0112, P0113, P0117, P0118, P0125, P0500, P0502, P0503, P1114, or P1115 are not present.
^ Coolant temperature is more than -40°C (-40°F).
^ Air temperature is more than -7°C (19°F).
^ Engine running more than 4 minutes.
^ VSS average more than 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 preset temperature is more than it should be.

ACTION TAKEN WHEN THE DTC SETS
^ First failure:
- The MIL is not turned ON.
- A DTC is stored in memory under Failed Last Test.
- The Failure Records are stored.
^ Second consecutive drive cycle with a failure:
- The MIL is turned ON.
- A DTC is stored in memory under history.
- The Freeze Frame data is stored.
- The Failure Records are stored.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after the third consecutive trip the diagnostic has run and passed.
^ The history DTC will clear if the fault conditions have not been detected for 40 warm-up cycles.
^ A scan tool can be used to clear DTC information.

DIAGNOSTIC AIDS
^ DTC P0128 will set if the diagnostic does not pass within 25 minutes after start up.
^ DTC P0128 is designed to detect a faulty thermostat.
^ It is possible for a skewed ECT sensor to cause a DTC P0128 to set.
^ A skewed IAT sensor may cause this DTC to set, The IAT sensor should read near ambient temperature.

TEST DESCRIPTION

Steps 1-2:

Steps 3-11:

The numbers below refer to the step numbers on the diagnostic table.
3. This step tests for excessive resistance in the ECT circuit.
7. This step tests for a skewed sensor through the range of temperatures affecting this DTC.