An engine runs more efficiently when hot. If a thermostat does not shut off properly when cold, it will let cold water pass & it will take longer to warm up. When travelling at speed in winter you may experience the temperature gauge dropping back to cold again. As thermostats age, they tend to open at colder temperatures. Most modern cars have an 82 deg C (180 deg F) thermostat, like the K11.
The OEM nissan thermostat has a rubber seal. This beaks down over time & becomes brittle. Pieces of it will eventually break off, which lets cold water pass. Long before this time, it will also probably open at colder temperatures and/or not seal when cold (eg. a 3mm gap when cold).
I was checking the operation of my engine using Nissan Data Scan software & noticed that the engine was not operating around 82 degrees when driving, it tended to sit around 76. The temperature did not get into the 90's when idling. After changing the thermostat on my CG10DE, the fan comes on at 97, off at 92.
Part number
There are two different physical sizes of thermostat used in CG motors. The Tridon book lists only one, Nissan Micra K11 CG10DE CG13DE as pt# TT239-180 My Japan assembled CG10DE required pt# TT241-180, 54mm OD.
Mid 2006 prices for thermostats in NZ as follows:
Partmaster NZ$18.63 + GST = NZ$20.95 incl GST
REPCO NZ$17.34 + GST = NZ$19.50 incl GST
Segedin Auto Spares, 379-385 Dominion Rd, Mt Eden. NZ$12.49 +GST = NZ$14.05 incl GST
Replacement
Thermostat Location
When replacing your thermostat, stick the thermostat into the head with a little sealant & wait for it to set. Then refit the cover, with more sealant. If you try to assemble it all at once, it is very easy for the thermostat to slip down out of place, & it will leak. You will also have a very good chance of snapping the bottom lug off the cap when you tighten the bolts & the cover is unique to the CG motor. Don't forget that the bleed hole in the thermostat goes to the top.