There is a problem with the transmission and you need to get it to either Honda or a mechanic.
Usually when the D4 light flashes it means there is a tranny problem.
Transmission error
The D4 light on a Honda Accord is the transmission warning light. These come on when there is a malfunction on the transmission. For the problem to be diagnosed properly the vehicle should be taken to a dealership.
The D4 light flashes indicating a malfunction with the transmission operation. You'll need to have it diagnosed by a mechanic with a scan tool to determine exactly why the light is flashing.
the d4 flashes when the ecu has detected a problem you will have to get the codes pulled from the ecu to diagnose the problem or switch off the ignition leave for a few seconds then restart should go away. Make sure the car is fully stopped between changes from reverse to drive.
Code 12 is a faulty EGR valve. You need to clean or replace the EGR valve of all carbon. The exhaust passage on which the EGR is mounted needs to be cleaned of carbon deposit as well. The flashing D4 light indicates a transmission problem. This needs to be looked at by a professional.
Locate the 2 pin diagnostic connector behind the glovebox door. (Not the 16 pin OBD connector which is on the driver side). Hook a jumper wire from one terminal to the other. Turn the ignition on, but do not start the engine. Count the number of flashes the D4 light makes, looking for both long and short flashes. Respond with the number, and I'll post the diagnosis. (there are more than 40 possible codes, too many to list here)
it means your motor is about too shut down and or is overheating.
It's the transmission computer.you need to replace it.
transmission trouble for sure
Replace your AT Control Unit (Automatic Transmission Computer)
A flashing D4 light on Honda Accords is commonly the result of a bad speed sensor. The speed sensor is located on the transmission and sends a signal to the speedometer and a signal telling the transmission when to shift.