Service engine soon light came on, what could it

If the light was flashing when it came on, then it means your engine is misfiring.

A solid light can mean anything from a loose/faulty gas cap to a transmission issue.

This light comes on when the computer detects a fault of some sort in its diagnostic tests. If it keeps detecting this issue, the light will remain on. If the light turns off, then it has detected that the issue may no longer be present

In order to find out what the exact reason was for your light, You need to have your car computer scanned to retrieve the code that the computer would have stored when the light triggered. Most garages will do this free of charge. The code will tell you what may have caused the light to initially turn on.

Theres a thousand reasons that light could be on. Get a OBD2 code reader form a parts store, get the code, then get back with us.