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 the ECT 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.

CONDITIONS FOR RUNNING THE DTC
^ This DTC will only run once per ignition cycle within enabling conditions.
^ DTCs P0112, P0113, P0116, P0117, P0118, P1111, P1112, P1114, or P1115 are not set.
^ The coolant temperature is more than -40°C (-40°F).
^ The coolant temperature is less than 78°C (172°F).
^ The intake air temperature is more than -7°C (19°F).
^ The engine is running more than 2 minutes, and less than 30 minutes.
^ The vehicle speed is more than 24 km/h (15 mph) for more than 1.6 km (1 mi).
^ The mass air flow (MAF) average reading is more than 15 g/s.

CONDITIONS FOR SETTING THE DTC
The length of time for the engine coolant to reach a preset temperature of 80°C (170°F) has been exceeded.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Clear the MIL and the DTC with a scan tool.

DIAGNOSTIC AIDS
^ DTC P0128 is designed to detect a faulty thermostat.
^ A skewed ECT sensor may cause a DTC P0128 to set.
^ If a DTC P0125 and DTC P0128 are both set, a skewed ECT sensor is the most likely cause.
^ A skewed intake air temperature (IAT) sensor may cause this DTC to set.

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.