P0128 on a Jaguar X-type 2004

Well this is a very common code that a lot of you X-type owners are going to get at some point. The fix is fairly simple, although it requires a bit of work and patience. P0128 is flagged by the computer when it "thinks" there is something wrong with the thermostat.

Here is what's happening: every time you start the engine from cold (below 95 degrees F) the computer monitors the coolant temperature sensor to see how fast the engine warms up. Remember that an engine runs more efficiently warm not cold. When the engine doesn't warm up fast enough two times (this code is a 'two trip MIL') that very hated check engine light comes on and P0128 is flagged.

Possible causes are:

• Coolant temperature sensor, I never changed one in this car in my life
• Thermostat, changed about 20 this year alone.
• Here is a link for just the thermostat: Stant 48788 XACTStat 180 Degrees Fahrenheit Thermostat

Changing the thermostat is my fix. In my experience the code never comes back after that. The thermostat could come as an assembly with the top radiator hose, which leaks all the time. So changing it will actually be preventing another very common problem. Here is how to change the hose: Changing the upper radiator hose on your x-type. Here is what you are changing... 

Here is a link to the radiator hose: Jaguar OEM X-Type Top Radiator Hose

HINT: if you are changing this hose I recommend to take the studs out of the thermostat after removing the nuts with a 5.5mm or a 5 mm (I don't remember of the top of my head) 1/4 inch socket and ratchet.