Speedometer not working - check the OBD port.

Just a heads up in case it happens to you - if you run into an issue where the speedometer stops working, make sure there is nothing connected to the OBD port under the dash.

For several months I've been driving my car with a bluetooth diagnostic adapter plugged in (I've been having intermittent issues with the automatic transmission controller which seem to be caused by electrical noise on one of the transmission speed sensor inputs), and today, the speedometer decided not to work - the odometer was readable and not changing and the speedometer needle remained at rest.  The transmission controller also reported a 38 error, which the service manual defines as a vehicle speed sensor open circuit.

Whilst printing selected pages from the manual to take out to the vehicle to troubleshoot the problem, I noticed that the vehicle speed sensor circuit was wired to the transmission controller, the speedometer, and for some unknown reason, the OBD port. 

Unplugging the bluetooth adapter solved the problem.