Code P0128 what it means and how to fix it

The OBD-II trouble code P0128 indicates an issue with the coolant thermostat and/or the engine coolant temperature (ECT) sensor. It means that the engine is not reaching its operating temperature within a specified time frame, or that the temperature does not reach the expected level. Here are the steps to diagnose and potentially fix this issue as a mobile mechanic:
Diagnosis:
1. Use an OBD-II scanner to retrieve the specific trouble code P0128.
2. Check the engine coolant level to ensure that it is at the proper level.
3. Inspect the coolant thermostat and the ECT sensor for any visible signs of damage, corrosion, or malfunction.
4. Test the ECT sensor using a multimeter to measure the resistance and voltage output. Compare the readings to the manufacturer's specifications.
5. Check for any coolant leaks, air pockets in the cooling system, or issues with the thermostat that could affect the engine's ability to reach the proper operating temperature.
Repair:
1. If the engine coolant level is low, top it up to the correct level and check for any leaks in the cooling system.
2. If the ECT sensor's resistance or voltage readings are out of specification, it may need to be replaced.
3. Inspect and test the coolant thermostat to ensure it is opening and closing properly. Replace it if it is found to be faulty.
4. If there are air pockets in the cooling system, ensure that it is properly bled to remove any trapped air.
5. After making the necessary repairs, clear the trouble code using the OBD-II scanner.
6. Conduct a road test to verify that the check engine light does not reappear, indicating a successful repair. 
Our mobile mechanic team, it's equipped with appropriate diagnostic tools, such as an OBD-II scanner and multimeter, to pinpoint the issue accurately and ensure the correct repair is undertaken. If you're unsure about the diagnosis or repair process for this code, it's recommended to consult with the vehicle manufacturer's service literature or seek assistance from a qualified automotive technician.