Volvo Repair: tranny, power flush, transmission oil

Question
QUESTION: hi roger,i wanted to know if i can just change the transmission oil myself or is best to have it flush.Can you give me an estimate on how much it will cost to flush out the transmission oil.Also the check engine light and the arrow comes on at the same time but when i depressed the button on the shifter it goes away why is that?Thank you

ANSWER: Luis, which year and model? You forgot to tell me that.  Roger

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QUESTION: 1992 940gl volvo.

Answer
Luis, its best to do just a drain and fill, then drive about 1000 miles, then do it again until the fluid is nice and red.  Do NOT power flush!!!! You are asking for trouble with that.  As far as the arrow, sounds like you need a new Overdrive relay.  Roger