Note that the job should be carried out when the engine is stone cold, as during the process a small dribble of coolant will be forced out of the system under pressure and the last thing we want is for this to be boiling hot and scald you!
The job takes about 10-15mins total and the only tools that are required are a flat headed screwdriver. If i can do it, so can you.
Issues with the coolant temperature sensor are fairly common on our cars and can often result to poor running when cold, fluctuating idle rpm and ultimately a check engine light.
There are to versions of the sensor available, the original "black" version which are prone to failure, and an updated green version. The part number for the newest version of 059 919 501 A. A plastic retaining clip as well as a rubber O-Ring are also required for fitting the sensor, these will often be included, but be advised to check
The OEM part will cost about £25-35 from VW, however in this instance i purchased a Febi-Bilstein sensor for about £10 - they are OEM manufacturers after all so shouldn't be any issues with quality. One word of warning, there are a lot of "no name" versions of the sensor available on Ebay - I would be inclined to avoid these.
On with the show:
1) Remove both the upper and lower engine covers (the two large lumps of plastic covering the engine area under the bonnet) The larger top cover is held in place by four press screws in each corner, apply a little downward pressure and a quarter turn will loosen them off. After removing the top cover, do the same with the lower cover, which runs along the bottom of the engine and alongside the cylinder head, same type of screws, but this time only three to be undone (the third screw is only visible with the top cover removed).
2) As the sensor we are replacing is on one of the coolant lines, we don't want all of our coolant to splosh out when we remove it. To prevent this, remove the cap from the coolant reservoir on the drivers side of the engine bay in order to release any pressure, then replace the cap to create a vaccum.
3) Locate the coolant temperature sensor, this is just to the right of the cylinder head. It is positioned at a 90 degree angle from one of the coolant lines. It is only really visible with the lower engine cover off:
4) As you can see, the sensor is held in place with a small plastic "U" shaped clip, this will need to be removed. There should be sufficient daylight for you to slip a small flathead screwdriver between the clip and the pipe and just ease it off. Apparently these are prone to snapping and breaking, however mine came out without issue.
5) Prepare your new sensor, by placing the rubber o-ring on the end. Some advised greasing this ring up with petroleum jelly, however i didn't have any to hand on the day.
6) With the retaining clip off the old sensor, pull up gently on the electrical connector and the sensor should come free. There will be a small amount of coolant that will spill forth when you do so, but if you followed the advice re: the coolant cap it should be an insubstantial amount.
7) Typically, like mine did, when removing the old sensor the old rubber O-Ring will be left inside of the socket you have removed the old sensor from. Reach inside and hook it out with a finger
8) Examine the sensor and you will see that it is connected to a an electrical connector, with a retaining function. To release the old sensor, use your flathead screwdriver to gently lever the small square tab at the rear of the connector. There should be a light click to let you know it has released. A gentle pull on the sensor should pull it free of the connector.
9) The rest of the process is just a reversal of the above procedure. Insert the new sensor into the electrical harness, again there should be a soft click to let you know it is locked in.
Replace the new sensor with O Ring back into the socket (the small clip on the electrical connector should face towards the rear of the engine bay). Again there will be a small click to let you know it is properly inserted.
Reinsert the U Clip to retain the sensor, there are a couple of holes through which to insert the clip into, the closed end of the U should face towards the front of the engine bay.
Replace engine covers
Job Done! Fire up the car and take it for a short test drive to check all is well!
Edited by BigJakk, 23 July 2012 - 00:23.