The system will still function as though it is coming on, although your compressor won't engage. As far as the cooling fans, I am not sure if they still come on or not. A faulty thermostat doesn't necessarily mean that the engine is not reaching operating temperature, it may lie in how long it takes to reach. I have included the literary portion of the diagnostic trouble code that may explain further.
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the engine control module (ECM) for engine control, and as an enabling criteria for some diagnostics.
The air flow that is coming into the engine is accumulated. The accumulated air flow determines if the engine has been driven within conditions allowing the engine coolant to heat up 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 ECT as enabling criteria, may not run when expected.
If engine coolant fails to reach a present target temperature before a calculated air flow is accumulated DTC P0128 will set.
This DTC is continuously monitored during engine operation.
CONDITIONS FOR RUNNING THE DTC
DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, or P0118 are not set.
The engine block heater has not been detected or delayed for 20 seconds.
The engine off time is more than 15 minutes.
The engine speed is more than 960 RPM.
The engine coolant temperature (ECT) is less than 50 C (122 F) at start up.
The ambient air temperature is more than -39 C (-38 F) and less than 50 C (122 F).
The vehicle speed is more than 15 km/h (9 mph).
The mass air flow (MAF) average reading more than 23 g/s.
CONDITIONS FOR SETTING THE DTC
The time for the engine coolant to reach a set 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.
Clear the MIL and the DTC with a scan tool.
DTC P0128 is designed to detect a faulty thermostat.
A skewed ECT sensor can cause a DTC P0128 to set.
A skewed IAT sensor can cause this DTC to set.
Wednesday, February 9th, 2011 AT 5:21 AM