I have a 2002 Honda Odyssey. Generally runs great but recently haven't
been able to get much heat out of the vents even when I set the temp to
28 degrees C. It also takes forever to defrost the windshield.
Recently, when I start up the van and it's idling, I can smell a faint
gas/oil smell coming from the vents which I never smelt before. It
goes away after I drive away though.

My mechanic thinks it's the thermostat so I'll get that replaced first
I guess. Coolant was recently replaced and it's topped up. Also cabin
filter was replaced. Temperature gauge on the dash says that the car
is in the normal operating temperature range. Could these be the
symptoms of something else other than the thermostat?