Well, there is only one cabin temp sensor and because the driver's side is cold, you are looking a a possibility that the temp dial in the ECC unit is bad. A Volvo scan tool can interrogate the ECC unit and should even tell the operator what the actual temp settings are on both sides. Unfortunately, most dealer mechanics don't know how to play with the ECC unit. Dealer logic is to throw in a new unit and hope for the best, with you paying for it.

Check Ebay and other sites for a used one. Pay particular attention to the face design of yours, the replacement must match exactly. There are two versions, and possibly multiple years.

If you are an electronics heavy, checking the temperature dial internals might be possible.
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My back feels better when I sit in a Volvo seat