Help please--dual shower heads

[WorldsWorstPlumber]

Hello all,
This is my 1st post here and hope someone can help me. About 1 year ago, my wife and myself hired a contractor to completely remodel our master bathroom. They've since gone out of business & I can't contact them. My problem is this...they enlarged our shower and installed a 2nd shower head at our request. Up until about 2 months ago, everything worked fine. The problem is with the original shower and not the one they installed. For about the first 8 months or so, everything worked perfectly. Then, the original shower started very slowly losing temperature...it went from hot to warm to cold. No matter how long you leave it run, it just doesn't get hot anymore. They are both on the same water heater so I think that can be eliminated as the problem. The manufacturer of the unit is Price Pfister. I was told by a co-worker that if I took the handle off (it's a single handle that rotates to supposedly adjust the temperature from hot to cold) that I would see a screw in there that I could turn to adjust the temperature. I got the handle off and the surrounding hardware but see no place to adjust anything. Any help would be greatly appreciated. Here's a photo...I hope this helps & I will sincerely appreciate any advice.

 

[Jadnashua]

The tab on the white part probably runs up agains a stop. It may pull off and you can rotate it a little then reinstall on the splined shaft to adjust the limit. You should be able to verify this on their website - they usually have the instructions posted.

Keep in mind that around this time of the year, the incoming water is nearly the coldest it gets, and that may be part of it. This is one reason why I like a temperature controlled valve - it doesn't have a user controlled stop in the same manner - you turn the thermostat and it then controls the water mix, rather than you setting a stop on the water valve itself. The more common mixing valve limits assume the incoming water temperature is the same. That may be true with the hot, but is only true in a few places in this country (maybe Hawaii?). Where I live, the cold can vary from 33-70 degrees...the limit would scald you potentially in the summer if set in the winter, and freeze you in the winter.

 

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[WorldsWorstPlumber]

Thank you Sir...

...but I still can't figure it out. It doesn't pull off easily and I'm afraid I'll break something. Also, I can't find the website you're referring to...if you have the time, I would greatly appreciate it if you'd post a link to it here.
Thank you,
Mark (The World's Worst Plumber)