03 Hyundai Accent check engine flashing????

It's been a slow coming problem thats reoccurring. I took my car to get emissions done
and a cylinder three misfire came back and also the check engine light didn't work at all.
I took it in and had them check the bulb and it was obviously broken, got that fixed by
them because I didn't want to do it myself. They thought it was the coil pack, we changed
it, re gapped new spark plugs checked the wires etc. A slight change but the check engine
is still blinking. The battery is pretty old and corroded, I was thinking about changing it and
seeing if that was my reason for the loss of power, it also won't read on an OBD reader
whatsoever. Im going to check the fuses again, see if I mess around with those. I'm at a
loss and Im not going to keep throwing possible parts that MAY work at it and end up
spending more than the car is even worth at this point.