1999 Cadillac Deville erase code

If everything got fixed, those messages would go away on there own. The coolant level may be a stuck sensor, the steering/stability system fault, idle fault may be emissions related.

Like I say, if your coolant is full, and you get a low coolant message, it could have a bad sensor that is part of the reservoir. Your P.S. Gear, pump, and such have little to no relevance on the service steering message UNLESS, the magnasteer actuator(on the gear) is electrically disconnected, or routed improperly to where it chaffed or burned through on the EGR pipe. Another possibility could be a disconnected or faulty P.S. Pressure switch that is on the pressure hose, this might cause the idle message as well, since the ECM uses it to adjust idle speed in response to steering load. If you could gather all the codes and report them back, I should be able to provide some further analysis.