My Focus warms up really quickly compared to other cars I've owned, but also intermittently the temp guage goes to the red, then back to normal. The heater is hot, but not over-the-top, but when I plug in my diagnostics the engine is running at 114 degrees celcius, when driving it goes to 134 degrees. It's recently had a thermostat fitted, but when the waterpump was checked it looks pretty new - only had the car for a month so not sure when it was done. It never loses water and there is no pressure build-up in the expansion tank. Has anyone got any ideas what it could be?

Could it just be something as simple as a defective temperature sensor?

Could it just be something as simple as a defective temperature sensor?

first there should be a build up of pressure in the expansion tank as the system works on pressure not a lot but there should be some pressure.

In order to prevent the coolant from boiling, the cooling system is designed to be pressurized. Under pressure, the boiling point of the coolant is raised considerably. However, too much pressure will cause hoses and other parts to burst, so a system is needed to relieve pressure if it exceeds a certain point. The job of maintaining the pressure in the cooling system belongs to the radiator cap. The cap is designed to release pressure if it reaches the specified upper limit that the system was designed to handle. Prior to the '70s, the cap would release this extra pressure to the pavement. Since then, a system was added to capture any released fluid and store it temporarily in a reserve tank. This fluid would then return to the cooling system after the engine cooled down. This is what is called a closed cooling system.

so change youre pressure cap first and see if this is youre problem theyre only a few quid

I have to say I don't think the OP's description fits a pressure cap problem. If the system is not pressurising and his temperature really is reaching 114 Celsius his coolant would be boiling off and he would see some coolant loss.

I'm going to order a CHT sensor to start with, then if there's no change I think I might buy a new water pump in case this one is faulty.