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A flashing overdrive light is a warning indicator from the computer telling you there are trouble codes stored in relation to the transmission which can be seen when using a scan too. The codes will help in diagnostics to help you determine if its a simple concern like a solenoid which is bad or a concern with the pressure being a valve body gasket blown out. More than likely its going to be an internal concern which a clutch piston slipping from worn seals or a band not holding from a servo piston seal worn.
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