This is an info post for future searches.

There is very little info available regarding this particular code and it took a while to get a straight answer as to how to solve it.

The transmission has two Speed Sensors (not to be confused with the sensor on the diff)- one input and one output. Basically, the input shaft speed is measured, multiplied by the current gear, and then compared to the output shaft speed. This is a diagnostic circuit that is continuously monitored to ensure the trans is performing correctly.

Anyways, the P0721 code indicates a malfunction in the Output Speed Sensor Circuit. The symptoms, aside from the check engine light, include lockout of the overdrive gears and a hard shift into second from first.

The solution is to simply replace the sensor. You can get the sensor for about $30. I replaced the sensor and the code came back after clearing it. Drat. I took it to the dealer, and after two days they found a short in my wiring harness. Luckily it was covered by warranty.

I hope this helps someone in the future.

 

very good info frog thanks for sharing.... im sure it will come into good use im glad you also got it taken care of thru your warranty.....saves a headache im sure