" It it's below 86 after running more than 10 mins, the thermostat is bad. "

Not 86 but 82 Is reasonable in colder weather with stop and go traffic. Highway 86 is not a problem though.
I've changed the thermostat twice (free fcpeuro replacement) to be sure. I had tested the new one before putting it in hot water with slow heat and it checked out around 88C so it wasn't bad just depends on your driving pattern.