Not sure if the sensor on my focus engine is in the same place on the fiesta/puma 1.6, probably not, but thought this might help. This is a diagram of the 1.6 in a focus, the temp sensor is by the spark plugs, number 4 in this pic
In the puma 1.7 its in the water outlet under the coil pack. Heres a pic, sorry its not very clear, its lined up next to other spare engines i have, but if you find the coil pack, you should be able to make out where it is.
My focus 1.6 doesent have any sensors there, so youll have to let us no if you have any sensors there or not.
On another note, if your car has 2 lambdas (normally a blue plug and a green plug), then if one was gone, there would definitely be a fault code. The fault code is generated when there is a either not enough difference between the 2 readings (they go either side of the cat, so this would indicate a faulty cat), or if the difference is too big, indicating on of them is at fault, and its highly unlikely that both lambdas would go at the same time. Are you sure it has 2 lambdas??
If its a 2 lambda model, that leaves faulty injector, or fuel pump in the fuelling system, although the ecu would pick up a faulty fuel pump as a lean mixture. It could be 1 or more faulty injectors, all the ecu would do is overcompensate for the faulty injector by increasing the pulsewidth of other injectors, it only knows a collective air/fuel ratio, so if one cylinder is lean, but others are slightly rich, the ecu wont know.
next cheapest thing would be a compression test, if its not a fuelling issue, and its not a spark issue, then the next thing is low compression.
Im running out of ideas here!!! :?