2004 S60, 127K, ECC, left side temp rheostat doesn't change temperature until all the way to the right into the red zone. Right side works fine, temperature modulating just as designed. Any thoughts?

Try setting the right side to different temps( blue / intermediate / red ) and see if it affects the behavior of the left side.

I say this because I recently figured out something with the controls on my 03 V70. Example: If the right side is set at red, then the left side wants to deliver only full-on or full-off heat. (Full-off if set to blue and full-on if set to any of the intermediate temps or red.) However, if I set the right control to anything less than red, even if only one notch below red, then the left control will give me cold at blue, variable temp at any of the intermediates, and full hot at red, as it should. Of course if I set the left side at full red, then the RIGHT side will not be variable - just full on or full off.

I don't know if that's how it's designed, but that's the way it works. Now that I understand it I'm fine with it. For the longest time it drove me crazy as the wife always wants to crank the right side to red which forced me to continually toggle the left knob between blue and the next notch up. I knew that wasn't right.