The ambient temperature sensor is located in the left side foglamp housing (lower drivers side of the front bumper). I'd start by checking that it's still there and plugged in. It's possible the carwash may have dislodged something.
As an aside, the temperature reading isn't particularly accurate. There's a thread or two about it on the forum.
As an aside, the temperature reading isn't particularly accurate. There's a thread or two about it on the forum.