I have a peugeot 106 and the temperature light stays on. I have changed the thermostate and the sender unit, checked all of the wiring and had a diagnostic test on it and still can not find the fault. It has coolant in it and everything is working correctly, but because the light is on all the time the fan is on constantly. Can anyone help?

I would check the temperature sender circuit. Either you have a bad sensor or a grounded wire.

the oil sender unit is screwed into the main oil galley of the engine block ans is in fact either a variable resistor for oil gauges or a pressure switch for oil lights
It normally has one wire from the gauge or light and is grounded out through the body of the unit
if the oil light stays on it indicates either a faulty sender unit or the wire is grounding out before the sender unit
do not confuse this unit with an oil sensor unit which has more wires and communicates with the ECM to report temperature and oil levels and pressures ( depending on the make of vehicle ) that will put on the MIL light or place the engine into limp mode

Normal to COLD?
You would only lose coolant if the engine was overheating - or, as you say - a leak.

It's probably a faulty temperature sending unit - these are common failures. You actually have nothing to worry about. The unit sends a signal from the engine to your temperature gauge in the car. Even if the unit is faulty the engine is still operating at the correct temperature - it's the signal to the temperature gauge that is wrong.

Your temperature sensor will look something like in the image below:
It's inexpensive and quick to rectify. A new sender unit can be fitted within minutes. Give a mobile mechanic a call ..

I read your question about the car heater.. No, the sensor wouldn't cause the heater to blow cold air. That fault is either air in the cooling system/heater system, or a blocked heater matrix.

Remove your radiator cap/coolant bottle cap. Open the heater controls to HOT and set on maximum. Start the engine and let it tick over - see if it forces any air out of the system when the heater should begin blowing hot.

If the heater still blows cold it may have a blocked matrix. Try a bottle of radiator/coolant system flush to see if that dislodges the blockage. The instructions will be on the bottle of coolant flush

I'm not a Peugeot expert, but do you know if the engine is warming up normally ?
The thermostat could be stuck open, or the engine could be low on coolant, or the sending unit for the gauge may have failed. If the engine is warming up you should be able to ground the wire to the sender and watch the gauge move.

fuel problem thats for sure ,nice motor this one compared to 5.7 litre american ,decent fuel economy and cheap to run ,now firstly i would remove the tank sender unit and using a torch look inside the tank for plastic bag or water anything that can block the gauze filter ,then fit two non return valves as close to teh pump as possible ,one in the inlet pipe and one in the return (injector pump) and see what happens ,while the sender unit is out check the pick up pipe as well for any splits or deterioation

it could be a couple of things, first, check the wires to the temprature sender unit make sure they have not broke, as it was sudden this may be it, also it could be the sender unit it self as a freind of mine had with her car i changed that and the one for the radiator switch, but most of the time it will be something to do with the sender unit on the engine...hope this helps

under rear seat , take out there is a big black circl, screw driver out its 1 side or the other

It could be a faulty oil sending unit or sensor. If the light sometimes flickers it could be worn main bearings at the crank shaft. But, replace the oil sending unit first that should take care of the problem.

check the fuses by the battery ,the big ones,then the other fuses ,if this fails then don mini and heels and drive to garage

maybe a fuse or a wire got disconnected from the guage