Code P0128 on the 2003 Chevy Trailblazer means that the computer has detected that the engine has not reached the normal operating temperature within the set time frame. This time frame is determined by the temperature of the air entering the engine, the temperature of the engine coolant, the speed of the vehicle and how long the engine has been running. The are generally no symptoms related to code P0128 aside from the check engine light.
If the engine coolant temperature sensor is faulty, it will relay false information to the computer, which may trigger the P0128 trouble code. The computer sends a five-volt reference signal to the sensor; as the sensor warms up, the resistance in the sensor changes, resulting in a different voltage being sent back to the computer. To test proper operation of the coolant temperature sensor, disconnect the sensor wiring harness, start the engine, place the ohm meter across the terminals and make note of the resistance. Plug the wiring harness into the sensor and allow the engine to warm up. Unplug the wiring harness once the engine is warm and check the resistance of the sensor again. There should be at least a 200-ohm change; if there is not, the sensor is faulty and should be replaced.
A faulty IAT sensor can also trigger changes in the computer that can cause a P0128 trouble code. To test the sensor for proper operation, disconnect the wiring harness for the sensor and make note of the outside air temperature. Check the resistance across the terminals of the IAT sensor. Compare the air temperature to the following values: At 50 degrees F, the reading should be around 58.75 Kohms -- 58.75 kilo-ohms, or 58,750 ohms. At 68, the reading should be around 37.30 Kohms; at 86, the reading should be around 24.27, and at 104, the reading should be around 16.15. If the resistance does not fall close to those values, the IAT is faulty and should be replaced.
If both the ECT and IAT sensor or operating correctly, the radiator cooling fan possibly is running too much, preventing the engine coolant from reaching normal operating temperature. The easiest way to test this is to start the vehicle and watch for fan operation while watching the temperature gauge on the dash board. The radiator cooling fan should typically turn on around 210 degrees and shut back off around 195. If the fan is running constantly while the ignition key is in the "on" position, there is an electrical short somewhere within the electronic fan relay or before the relay, causing the relay to always stay energized, which allows the fan to continue running.
If the radiator cooling fan is working properly, and all of the listed sensors have tested as good, then chances are the thermostat is stuck open, allowing too much coolant to enter the engine, essentially allowing the engine to run too cool and setting code P0128. Replace the thermostat and any coolant lost during the replacement and clear the trouble code.
