If the D4 light is flashing the computer in your car sees a transmission problem. Trouble codes are read using a scan tool attached to a diagnostic connector all the way to the right lower side of the glove box. Codes can be "read" by bridging the connector and counting the blinks of the D4 light on the dash with the ignition key turned on, but there are three connectors there connecting the wrong one can do damage. There is a company that independent auto repair shops "buy" their information from that offer a very reasonable service to do it yourselvers. You can get a one week subrcription or year subscription very cheap. The information is year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... great quality information same as the professionals have ,much better than generic workshop manuals from parts stores, much cheaper than the factory manual try http://bit.ly/mitchell_repair_manuals_online and get an online repair manual subscription.