1998 Other Chrysler Models engine trouble codes P1684&p0

Check engine light on while driving. Checked for codes by turning ignition to the on position three times. The odometer flashed p1684 then six dashes like this - - - - - - then flashed p0551 followed by six dashes - - - - - - then the word DONE.I have found that p0551 is power sterring pressure sensor circut range-- performance. Can not find any info on code p1684. Can you help? This is for 1998 chrysler intrepid with a 2.7 litre engine. Thank you ron