Cabin Heater Doesn't Control to Temperature Setpoint

Hey all,

My cabin heater doesn't control to the temperature setpoint. There's no difference between "65" and "High". It gets pretty toasty after a short while so I end up regulating manually by turning on and off periodically, which is slightly annoying.

Any ideas how to fix it?

Am thinking it's a faulty temp sensor, but it appears that there are two in the cabin: the cabin air temp sensor and heater temp sensor. Anyone know which is the relevant one and where it's located? I assume it's the latter but thought I'd check-in here before ordering parts.

Thanks!
Warren