2,4 indicates a bad CMOS battery. the 2,3 code indicates either a bad motherboard, memory, or CPU as shown on the chart here. Try reseating the RAM and if that does not help then pull all the RAM except one RAM module and try again. Do that with each RAM module to see if the system eventually boots properly. If that does not work then try a known good RAM module. If that does not help then possibly you may be looking at replacing the MB, or CPU or RAM.
The amber diagnostic light flashes 2-3, I have tried replacing the CMOS battery but still getting the same problem
Dell Optiplex 3020 won't boot