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I suspect the cold water side is clogged with sediment. When you turn the water off then back on again (when this was installed) it can stir up the sediment that clings to the inside of the pipes. You'll have to pull the thermostatic cartridge then the dirt strainer behind it. Also flush out the lines while you have the cartridge out. Here's a link to the product page. Click on "Spare Parts/Exploded drawing to see how this goes together. Of course you'll need to turn the water off before pulling this apart.Product page