Check engine light is on and the R for reverse on the dash panel stays on even though you are in drive?

1998 toyota camry check engine light came on and at the same time the R for reverse on the dash panel came on even though it was in P (park), when you shift gears to drive D, the reverse R light stays on. What causes this? Added note: have had to replace the radiator several times for unknown reasons, after this last time it was leaking tranny fluid at bottom of hose to radiator and over heating. Replaced radiator again, no more tranny fluid leakage, but still overheats occassionally after approx 20 min of driving on hiway(never in town)but goes back down to normal temp on its own after a few minutes. There is no pausing, hang up or slippage that we can feel in the tranny. Approx 150,000 miles. What is going on? Thanks!