Dodge Magnum Check Engine Light on, code P0128

When the check engine light turns on, it indicates that the computer sees a fault.  When the engine computer sees the fault, it will store a code.  The code will direct the repair technician to an area to test.  Today I have a 2005 Dodge Magnum with a P0128 code.  This code is basically the same for any make or model.  This is a standardized code for O.B.D. 2 cars and trucks.  P0128 is for Engine Cooling System Thermostat Rationality.  The engine computer (PCM or ECM) predicts what the engine coolant temperature should be, based on the engine coolant temperature at start-up.  The predicted engine coolant temperature is then compared to the Engine Coolant Temperature Sensor (ECT)  The error between the two is calculated and integrated with respect to time.  When the Thermostat diagnostic runs, the integrated error is compared to a calibrated threshold and a pass or fail is then determined.  Many things can cause the code to set, but they all have one thing in common, Engine Temperature.  Any electric or mechanical component that can affect engine temperature readings will cause the code to set in the computer, even a faulty computer.

The image above is from the Dodge Magnum.  The black rubber body around the center disc failed.  (new on left vs old on right)  The rubber is in place to make a tight seal around the outer body.  This rubber sealing ring keeps coolant from flowing untill the engine reaches the correct temperature.  If coolant flows too early, the code will set.

The engine computer is looking at the coolant temperature to adjust timing and fuel delivery for best economy.  A faulty Thermostat can cause reduced fuel economy, poor heater operation and hard shifting from cars with automatic transmissions.

It’s all good under the hood. Anthony Xavier ASE Master Technician

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