Well, it could be a control valve, etc... If it's the t-stat, they
should be able to see abnormal temp gauge readings. If the
gauge reading is the same it's ever been, IE: appx 1/3 range
seems about normal, I wouldn't worry about the stat. If the
stat was stuck open to reduce heater temp, they will see a
colder than normal temp gauge also. Most "no heat" problems
are the water control valves, stuck/broke cable, etc. An open
stat would give cooler heat than normal, but it wouldn't be totally
cold.  My accords temp gauge stays at about 1/3 range all the
time. Never moves once warmed up. I assume I have a stock
temp t-stat, but I've never actually looked at it yet. I'm not
even sure what the stock t-stat temp for a honda usually is.
I assume the 190-195 degree range most likely. My ford trucks
stock stat temp is 195. BTW, with my 1/3 temp gauge, the
heater will fry you out of the car if you turn it all the way up.
Once warmed up, I'll have to turn it way back nearly to the
beginning of the red line if I don't want to roast myself. :/
MK