Recommend complete flush and new filter. Not very hard job for DIY person. Lots of info on the sticky post at top of page for T6 trans service.
My 2004 T6 has 126,000 miles of trouble free transmission miles. Preformed complete flush around 90,000 miles and went with Redline D4 ATF fluid.

Have 2004 T6 with 125,000 miles.
Never had problem with motor or transmission

Sounds like you may have small coolant leak in or near heater core.

Thanks for the suggestion, a resistor sounds much cheaper than the sensor to replace. Does anybody know where I can find a step-by-step guide to replace either the resistor or air quality sensor?

Thanks again!
For fan and resistor replacement go to page two here, see post "Anybody replace blower ...

Majority time XC90 fan motor goes nuts, runs battery down by fan running with key in off position.
Blower motor resistor has gone bad.
Just replaced my blower resistor few weeks ago on my 04 T6 XC90. Not hard job to do yourself.

Have load test performed on your battery.
7 years close to a batteries service life.

Thanks
122K on my facory O2 sensors.

Replaced my fan resistor today. Not that hard of job, took me 2 hours. Dealer wanted 10 hours labor to take out dash.
Remove panel held in place with two screws located under dash with light hole in it.
Pull back carpet and insulation right side passenger floor. Just enough to get access to fan. Do ...

Looks like my blower resistor needs replacing. Proper way to replace blower motor and resistor is to remove dash. 7 hours labor.
Internet has few posts of blower being replaced without dash removal. Looks like you break off one of the blowers mounting tabs. 1 hour labor,
Has anybody replaced blower ...

Nice job, Thanks.