I had a grohe 34908 valve installed in June 2002. It worked fine for 11 years, but in 2013, the thermostatic cartridge failed (started leaking). It was replaced in Jan 2014 (bought Dec 2013) with a thermostatic cartridge (47582000) and it worked fine until Sept. 2015. At that time, it stopped providing hot water. After several temperature adjustments, the cartridge failed to provide any hot water at all, and was again replaced in October 2015. I also replaced both check valves at this time. It's now failing to provide hot water again, again after several adjustments. I have another cartridge that Grohe sent, but would really like to know if I'm going to be replacing these once a year. Does anyone have any information on what the failure mechanism could be that's causing these to fail so frequently? Thanks.

something isn't right.. grohe is an excellent product and their replacement parts are as well.

I agree, but it's happening. I see these described as "Paraffin cartridge". Does this mean that there's paraffin internal to the cartridge? Could this be failing/leaking in some way, which causes it to stop regulating?

I can't answer that since I've never encountered this problem.. perhaps someone at grohe tech support can answer that.
also ask them if they have anyone in the area who can do a site visit and have a first hand look. sometimes the local rep will enjoy getting out of the office for the morning and come out to have a look-see...