"Check Engine Soon" indicator

1999 Dodge Caravan - 85,000 kms.
Was driving today and noticed the shifting seemed to be a bit hesitant.
After making a brief stop, started driving again and noted the speedometer
was at zero, transmission would not shift out of second and within 2
minitues the "Check Engine Soon" light came on.
Drove home (about 5 min away) and just as I pulled into the garage the van
stalled and the "Oil Pressure Warning Light" came on.

Tried checking the engine codes (held trip reset and trip odometer switches
then switched ignition to on and held for 5 seconds).
It seemed to go through its diagnostic tests but the only code I noted seems
to be "999". Tried three times all with the same results.
Restarted the van and both the "Check Engine Soon" and "Oil Pressure Warning
Light" remained on.

Any thoughts or guesses as to what might be the cause of this?

tia.