Shower handle doesn't control temperature properly

Specifically, today I could only get HOT water. I had to turn it all the way to the right (cold), to bring it to a tolerable temperature.

Prior to this, setting about in the middle gave me my preferred temperature (all the way to the right was icy cold, to the left melt-your-skin-off scalding).

Today is the first time this specific issue happened, but I have been having trouble controlling temperature for some time. What typically happens is I turn it slowly left, which should gradually increase the temperature, but it doesn't - temperature stays the same until I've turned 90 degrees or so, then suddenly jumps too hot. Then the same thing happens as I slowly rotate to the right. I have to repeat this several times until I land on a tolerable temperature.

I'm guessing the 2 issues are related. Can anyone tell me what the problem is? And is it something I can fix myself (I'm fairly handy), or should I call a plumber?

The valve trim is Moen T8350CBN, the valve underneath was installed by the plumber who ran pipes when I remodeled the bathroom in 2014, I don't know if I can find a make/model for it.

 

Most obvious fix is to change the cartridge. Depending on how old it is and your water hardness, it could be an easy diy job or require a plumber. Took me about 3 hours of frustration and $30 for the cartridge puller. I was having similar symptoms.

 

So, this is what I see behind the trim. Am I correct in thinking that little thing poking up from the top of the cylinder is a retaining clip? And if I pull that out through the top, I should be able to pull the cartridge out through the front by gripping the shaft with a vise-grip or something?

 

Rotate the cartridge a full 180 degrees while you have the water turned off to the house. This flips the orientation of Hot and Cold. Then test your shower and see if the exact opposite happens where you have very little hot and too much cold.

 

In addition to the suggestions I'll add to call Moen customer service. I recently had the exact symptoms on a Price Pfister valve and found out they and Delta both have lifetime warranties. Moen may also. The Rep was able to narrow down the choices by asking the right questions over the phone. I had a picture on my phone and they had the ability to instantly receive it to verify my model. I received the free parts in about 10 days. One of the best customer service experiences I've ever had.

 

Yes, that thing poking out is the retainer clip. That’s the easy part. The cartridge could be a say or a nightmare to remove. Start ally in the day and prep for lack of water in th house.

Home Depot and the local Timber Mart in my area give Moen cartridges away for free. Just stop by and ask them.

 

I replaced the cartridge. It was super easy.

The old one was badly caked up with gunk. Works fine now. Thanks everyone.