My Bentley manual says that a 2-3-1 fault code is "Fuel system compensating for rich or lean air/fuel mixture at cruise." If you want to erase the fault code memory here's what you do:
1. With the diagnostic unit test lead in socket #2 and the ignition switched on, depress the diagnostic unit push-button for at least 5 seconds, then release the button. After 3 seconds, the LED should come on.
2. While the LED is on, again depress the push-button for at least 5 seconds. The LED should go out when the button is released.
3. Depress the push-button for at least 1 second, but not more than 3 seconds, a code 1-1-1 indicated that the memory has been erased.
4. Switch the ignition to OFF, remove the test lead and install the cover on the diagnostic unit.

...hope that helps...