The CJAA engine thermostat is inside a housing that is very difficult to remove. I do not suggest replacing it unless necessary. The oil filter / cooler assembly must be removed first. Looking at the parts diagram, the CBEA engine (2009 Jetta sedan, sportwagen, and Audi A3 TDI) appear to have a simple adapter over the thermostat and is like older cars. Replacement is much easier since it appears you only have to remove the adapter. Use the pictures here for refernce if you have a CBEA engine.
While not technically difficult, I rated it 3/5 because it's so annoying and time consuming to remove all the other stuff for access.
NOTE: This was performed on my demo engine out of the car. If you can provide additional tips, corrections, or pictures showing this procedure in the car, please post in the myturbodiesel.com TDI forums and help improve the article! This demo engine was partially paid for with site donations, please use the donate button above so I can provide more great DIY! In addition, the official service repair manual does NOT show this procedure. It only shows replacement as if it were an older engine or CBEA engine.
The housing is held with 2 bolts but you can't remove it unless you remove the rigid pipe to the right or the HPFP/AC/alternator bracket. Unfortunately, the bolt holding the rigid pipe is underneath the oil filter housing. Remove the oil cooler and oil filter housing, see 1000q: mk5/mk6 oil filter housing removal for details. If you have a CBEA engine you may not have to remove it. If you have a CBEA engine, please post your report in the forums so I can improve this article!
You may have to remove the alternator for clearance. This test engine didn't come with one so I'm not sure.
Remove the bolts and the hose clamp indicated below.
In the left side of the engine (sitting in the car), above the transmission is a single 12 point (triple square) bolt. Remove it to get play on the rigid coolant pipe and you now have enough play to push it around and get the thermostat out. You have to remove the air intake box for clearance. See 1000q: air intake removal for details.
Here is the thermostat inside the housing. Older engines (and from what I can tell, the CBEA engine) used a thermostat that was pressed against the block and had a simple adapter over it.
Clean the engine block sealing area of any dirt and coat the new o-ring area with water or coolant. Installation is the reverse of assembly.
Torque spec for the thermostat bolts are 11 ft-lb (15 Nm).
If you have more questions about removing the thermostat or can add pictures of this procedure while the engine is in the car, please ask in the myturbodiesel forums linked at the top or search below: