Hi Jimmy,
An I right in assuming you mean the engine speed sensor just on top of the ring gear.
If so, the sensor is held in place with a locating bolt on the side of the sensor.
Once removed, the sensor will just lift out.

Also D156 '' Traction control / heartbeat message missing for more than 400ms ''
Means engine speed sensor u/s.

I await your reply before we go any further.

Mick