If you are going to do this yourself do it with the engine COLD. Coolant operate at over 100'C so it will burn you!
I think on that engine the sensor and thermostat are built as a single combined thermostat housing. The easiest way to find the Thermostat housing is to follow the large, black coolant pipe that is coming out of the top of the radiator. follow it back towards the engine and the other end of the hose should be attached to the thermostat housing.
On the 1.6 it is usually on the transmission end of the engine between the cylinder head and the air filter box housing which you will need to remove. Once you have removed the air filter box access to the housing is much easier.
I have attached a video link as a guide for the job. The car in the video may be different to yours but the 1.6 is a standard Citroen engine so it is fitted to lots of different models.
This engine uses the cartridge style filter and Not the screw on type. It is positioned on the front of the engine.
*NOTE* Do this with the engine oil COLD as engine oil can reach 250'F (120'C) and will cause burns!
Stand with your knees against the front grill and look down between the radiator and engine block. On the right side, (your right), on the engine next to the exhaust manifold there should be a large round, black plastic cap with a large bolt head shape on the top. This is usually a 27mm.
It is plastic so don't use too much force when unscrewing it and you might need to turn it a lot before it lifts off. The filter has rubber seal rings on it making it tight but it should just pull up and out.
When refitting install by HAND first to avoid stripping the threads. Tighten until it just becomes tight then STOP. The cap does not need to be too tight as it is on the Low pressure side of the oil system.
I've attached a photo as a guide.
Question edited to add maker.
How much should I torque my brake caliper bolts?
Because caliper bolts MUST stay on, they need a LOT of torque. AutoZone, for example, recommends 113 pound feet of torque. To put that number in perspective, the average person can, according to NASA, apply about 15 pound feet with a bare hand.
Lots more here
https://www.google.com/search?q=Citroen+C4+Grand+Picasso%2C+brake+caliper+bolt+torque+settings
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start the engine then turn blower on full and then reach under the passenger side LHD vehicles and thump motor with a clenched fist ,the brushes often stick and thump clears it and it works again for a while ,new motor 196 euros plus tax
Normally you just go to the dealership with the registration document and they give you the code. You will have to get the code from a site based on the serial number. Although many of these say 'FREE', expect to pay $12 to $18.
https://www.google.com/search?q=philips+radio+code+generator+free
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