on u-tube a couple guys show a much cheaper 14 dollars fix for this not easy but can be done im tj and have blogged on this before,the car may run ok but gas mileage and will not pass emissions where applicable have had this problem multiple times it is listed under engine and drive train also
Problems for specific Cadillac Seville STS years:
Car problem reports
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Newest reported Cadillac Seville STS problems
The torque converter clutch (TTC) may stop working due to an internal transmission failure. The Check Engine Light will illuminate if this occurs. Our technicians tell us the transmission will require dis-assembly to correct this concern.
A Loose or worn gas cap may cause Check Engine Light to illuminate.
An updated PROM (Programmable Read Only Memory) chip may be available for the powertrain control module (PCM). Check with your dealer.
This powertrain control module (PCM) may develop a fault with its electrical ground location at the front of the engine block near the starter motor.
Excessive carbon build up in the engine combustion chambers may be the cause of a smog check failure due to excess NOx emissions. A "top engine" decoarbonization cleaning procedure will commonly correct this concern.
An intake manifold vacuum leak can develop from a variety of sources, often causing illumination of the Check Engine Light with codes P0171 & P0174 stored. Thorough diagnoses will be necessary to pinpoint the cause in order to make the correct repairs. A "smoke" machine is often used to isolate the cause of difficult to find vacuum leaks.
One or more heater & AC (HVAC) temperature and/or air delivery mode door actuator may fail, resulting in improper temperature or air delivery. If this occurs, fault code(s) should be stored in the HVAC control module which should assist in diagnoses.