Note On Intermittents
If all digits of ED71 and ED72 are "1", check an intermittent loss of ground in Park/Neutral switch. If an intermittent Code EO91 occurs when transmission lever is moved, the code is most likely caused by a misadjusted shift linkage or Park/Neutral switch, causing the Park/Neutral switch to end up in the range between gears.
If the code and SERVICE ENGINE SOON message comes on while driving or when gear selector is not being moved, record ED71 and ED72 and compare values to the chart. example, if transmission was in OD when message came on and ED71 reads 10 and ED72 reads 11, check intermittent open in circuit No. 773.
Manipulate related wiring and connectors while observing parameters ED71 and ED72 (or "Scan" tester display). If an intermittent is induced, the digit corresponding to that circuit will change. If wiring and connectors are okay, substitute a known good Park/Neutral switch and recheck.