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hello all,i have a 98 seville sts with some major problems,most of the time everything is fine then the dash will go completely out or the gauges will act all crazy and at the same time the engine runs and idles like crap and then all the sudden everthing is fine again sometimes lasts for a minute sometimes an hour,you can never tell,the recent and past codes are all the same,po121 tps sensor,po300 random misfire,po507 idle control rpm high,po741 torque converter ckt performance or stuck off,when all is cleared and is running right without the dash problem i get no codes until it does it again,its the same thing every time,i was told it could be a ground but these things i would think all have seperate grounds and still the dash going wild,the car has only 110,000 miles and looks mint but thinking about taking it over the scales and accept the loss,i have worked on cars all my life but for the world can not figure this one out,if someone has a clue all help is greatly appreciated even if you tell me to go ahead and have the piece of junk crushed,thanks in advance for any help,thanks dave........
 

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99 Seville STS, 08 STS4 N* 1SG F55
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To start with the flamingly obvious it certainly sounds like an electrical problem - usually a collection of codes combined with dash problems ends up with that diagnosis. To trace the root cause you will need to do some hunting. Try checking the battery voltage reading either on the DIC or with a meter when it starts acting stupid - if there is a noticeable drop in voltage you can start with the battery. Checking the battery connections for tightness and clean is a good safe start to begin with anyway. Was there anything done to the car right before this started? If there was a repair, checking the wiring connections in that area is another safe bet. After that I would recommend a factory service manual where you can check for common areas to the circuits associated with the codes in your post. If you can trace them to a common connector or ground you should also have a good lead on your problem. Let us know how you make out.
 
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