P0128 OBD-II Trouble Code of 2004 VW Jetta TDI

I always have the impression that usually diesel cars are not sensitive to throw out faulty codes until something is really going wrong. However, for the first time after owning it for two years, the yellow light is on with code P0128:  which means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The easiest part to replace is the coolant temperature sensor (CTS), which costs $55 bucks at the dealership along with the $5 O-ring. 

The original installed sensor is a black one and said to be easy to break. The improved part is green (see the above picture). To replace the sensor is not difficult but could be messy, since coolant will nevertheless run out during replacement. The CTS is located right below the vacuum pump at the diver's side of the engine bay. (See the second picture.)

The following is how the sensor sits in the car:

To access the sensor, it is recommended

Step 1: remove the intake hose and housing. 

Step 2: remove the electric connector; press the pin on the connector to release;
Step 3: insert a small flat head screwdriver below the sensor to release the clip which holds it.

Step 4: remove the old sensor along with the O-oring.

Note the O-ring may get stuck inside and cause trouble to install a new one.  Remember to search for it.

Note also that the connector may not be easy to release. In such a case, one may choose to reverse step 3 and step 2, allowing more room above the engine bay to remove the connector with the old CTS still on.

Step 5: Install the new CTS with new O-ring, and the new clip to hold them.

Step 6: Hook on the electrical connector.

Step 7: Add more coolant to the required level. Remember to recheck it after driving a few times.

Step 8: Remove the faulty code with a scanner. The code does not go away automatically.

Step 9: That is it.