I too have an S90 (97) and am just starting to learn all it's tricks and characteristics. I do know that the transmission computer fault codes can be read out through the OBDII connector and therefore perhaps a fault in tranny would light the check engine light. The check engine light may have been triggered by something that happened while cleaning the engine such as a cable knocked off or water into something.
In any event if you disconnect the battery cable for a while the light will reset and as long as the fault is no longer present it will not come back on. Alternatively you can have the fault code read out by a tech that is equipped with a code scanner or go to an AutoZone where they will read the code at no charge but will most likely refuse to reset it.
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David Hunter
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