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.
CONDITIONS FOR RUNNING THE DTC
* This Diagnostic Trouble Code (DTC) will only run once per ignition cycle within enabling conditions.
* DTCs P0112, P0113, P0117, P0118, P1111, P1112, P1114, or P1115 are not set.
* Coolant temperature is more than 40 C (-40 F).
* Coolant temperature is less than 78 C (172 F).
* Air temperature is more than -7 C (19 F).
* Engine running more than 2 minutes and less than 30 minutes.
* Vehicle speed is more than 24 km/h (15 mph) for more than one mile.
* Mass air flow average reading more than 15 g/s.
CONDITIONS FOR SETTING THE DTC
The length of time for the engine coolant to reach a preset temperature 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.
* Use a scan tool in order to clear the MIL and the DTC.
* DTC P0128 is designed to detect a faulty thermostat.
* It is possible for a skewed ECT sensor to 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 IAT sensor may cause this DTC to set. The IAT sensor should read near ambient temperature with the engine running.
It could be a bad coolant temp sensor or a bad inlet air temp sensor but is most likely a bad thermostat.
Please consider a to help us answer more questions.
Monday, October 25th, 2010 AT 8:44 PM