2004 s60 AWD. HEater blows cold air most the time, then warm, and back to cool. Temp guage at 12:00. Mechanic says it's thermostat and he measured temp at 120' and guage not right.
Had a new thermostat and housing 2 years ago (10,000 mi +-)
Thing is, temp switches quickly with no observable change in guage. Radiator fan continues working for short while after shutting off engine showing it's still cooling down.
I nsuspect something going on in the sensors or climate control. Don't want to put in another $365. Thermostat if not needed.
suggestions?

It only comes with a complete replacement housing. The part coast about $165. The labor was around $250 which I don't understand at all. Even if they have to remove the power steering pump to access it, it should take that long.
online search on total cost range from $250-$500. I'm tempted to try it myself.

i know the coolant temp sensor is malfunctioning as I frequently get the warning message for low coolant level, even when full. Does this feed into the function of the heater?

I'm surprised I can't find any info anywhere diagnosing the heater flap doors function. The heater will work briefly then back to blowing cool air rather quickly. Faster than I could imagine being affected by the thermostat.
This morning I will let thme car run and put my hands on the heater core hoses to evaluate. I can also measure the coolant temperature in the overflow tank with a thermometer.

No blinking lights.

Today I ran the car till temp guage read normal (12 o'clock). Heater still blowing cool air. Put a thermometer in the overflow tank and got reading of about 120'.
Hoses in and out of heater core very cool. Heater hose from thermostat to radiator hot.
Reved engine to 2k for minute or two. Heat began coming thru but returned cold when back at idle.
So, seems like heater core working, heater flaps working, but not reaching temperature. Looks like it needs another thermostat again?