Your temp sensor is likely faulty. There is a known issue with the denso temp sensor on the rear cylinder and it mis reports the temp to the ecu.
It was replaced by a bosch version.
If you look at the left side of the rear cylinder you will see the sensor. If it has a bluish plastic look then it's the denso part. The Bosch is bright Green.
When you buy the bosch part you have to buy a small wiring loom also as the connectors are different but it's quite cheap.
I'll look out the part numbers and post them up later.
ps. it's fiddly to get to but well within the home mechanics remit if your ok with spanners.