I have a 93 cadillac deville the OBD says the following code errors:
E26 - Shorted Throttle Signal Switch
E31 - Shorted MAP Sensor
E48 - Exhaust Gas Recirculation Sysyem Problem
E52 - PCM Memory Reset
E55 - Closed Throttle Angle Out Of Range
E75 - Intermettent VSS Signal
E91 - PRNDL Switch Problem (transaxel range switch problem)
E98 - High RPM P/N to D/R Shift

Which problems should I tackle first? How do I know where to start with all of this?

T, September 2010

First check battery then charging system, check earths. Note codes , clear codes see which codes come back, Use decent scanner to check live data on throttle pot, prndl switch etc.etc. It is unlikely to be all these faults. Its possible they have been disconnected with ign. on.

mags, September 2010