Thermostat Sensor Change

Taking the clip out is easy, putting it back in isn't as easy.

Place long nosed pliers or a screwdriver between the housing and the clip and lever the clip out.

It has been noted by some that it is a good idea to tie the clip to a part of the engine 
with string or fishing line as it has the ability to spring out and totally disappear 
during its removal and refitting. It saves the hassle of getting replacements.

Neil also recommends that you should be wary of the O ring that can come loose when you remove
the sensor. The expelled coolant can push the O ring out and you'll have a job finding it again.

The best way is to hold the clip in place, push the sensor into the hole and 
hold it in and use long nose pliers to lever the sprung clip back in place.
Be careful that you don't damage anything with your leverage.