1994 BMW 740 SMOG TEST FAILURE

I RECENTLY TOOK MY 1994 740IL FOR SMOG CHECK AND FAILED THE TEST AS FOLLOWS: IT SHOWS GOOD PERFORMANCE AND PASSED AT 25 MPH BUT FAILED AT 15 MPH IN ALL CATEGORIES (CO2, O2, HC, CO, N0). I DON'T SEE ANY SMOG COMING OUT MY CAR. CAN YOU GIVE ME AN ADVICE ON HOW TO SOLVE THE PROBLEM AND WHAT COULD HAVE BEEN THE ISSUE? ANY LEADS AS WHERE TO TAKE IT FOR REPAIR AND ESTIMATED COST? THANKS