Most Fs , this will be the brown sensor, in later Fs and Tfs it is black.
The ECM uses engine temperature to calculate fuelling and ignition timing parameters during start up. It is also used to provide a temperature correction for fuelling and ignition timing when the engine is warming up, running normally or overheating. The ECT signal is used by the ECM to control the engine cooling fans.
If the sensor fails or becomes disconnected, the ECM will use a default value which is based on values from the engine oil temperature sensor. The driver may not notice that a fault is present although a fault code will be stored in the ECM which can be retrieved using TestBook. The default value will also include operation of the cooling fans in fast mode when the engine is running.
REMEMBER: DONT DO THIS WHEN COOLANT IS HOT!!
1. First, you need to remove the engine access panel (I have not covered that here). Then locate the sensor. Looking from above the engine bay, over the cylinder block, notice the coolant pipes at the rear (LHS on the photo).
2. Here is a close look. You can now easily see the sensor (black), and its even more obvious if you have a brown one
3. Remove the multiplug from the sensor. Easy on the later ones, just press the middle of the clip and then pull. I have heard the older ones are a bit more tricky? AT THIS POINT you can now check if your radiator fan works. With multiplug disconnected, start engine. Rad fan should come on immediatley if it is working OK.
4. When you remove the sensor, you WILL lose some coolant, so best to install some tissue/towels etc undserneath the sensor to catch spilled coolant
5. Now start to remove the sensor. I dont know the exact size, only that none of my sockets would fit over the connector block part, so I used an adjustable wrench.
6. Loosen the sensor A LITTLE BIT. Coolant will start to leak soon, and you need to be ready with the new sensor and a spare thumb to block the flow.
Remove the sensor by hand and once clear of the hole, plug with thumb.
7. There should be a little black gasket which goes round the sensor. KEEP this and repalce over the new sensor (mine has disappeared for some reason so I need to get a new one).
Simply screw in new sensor and tighten. Replace multiplug. Remove tissue/rags and then replace engine bay access cover.