When mine shows "9999 MPG", it also shows "OPEN LP".
There's a section in the
ScanGauge manual for setting up fuel cutoff parameters:
"MORE>SETUP>FUEL>CUTOFF
Some vehicles will turn the fuel injectors off when coasting. This will
cause the fuel economy to go to 9999 MPG or 0.00 KHL. This shows
when you are not using any fuel while coasting. This is sensed through
the open/closed loop indicator and the throttle position. To tell if the
throttle is closed, the throttle position is checked against the CUTOFF
value. CUTOFF is the value the throttle has to be below in order to
indicate a fuel cutoff and show 0 fuel flow.
< 24 >
Cutoff SAVE>
The default is right for most vehicles, but it can be adjusted by pressing
the upper left or right button. The setting is not critical, but is best if
about 4 above the no-throttle TPS value. If set to "0" it disables this
method of fuel cutoff sensing."
Hope this helps.