GE Fanuc 90-30, CPU 351 Loses program

I'm working on a 90-30 PLC which keep losing its program everytime it is powered down.

We first figured it must be a battery issue, so we tried replacing it, no change. We then connected the battery directly in the CPU, rather than through the power supply, thinking that maybe the power supply or the back pannel might be defective, but still had the same issue.

Last, we replaced the CPU, but the new one is also wiped when the machine's power is turned off ...

Anyone got any clue for the next step ? I'm thinking maybe replacing the power supply ?

Thanks guys !

Check the CPU's hardware configuration. It could be set to read from flash memory on power-up and maybe there's nothing saved to flash.
Do you get a "low battery" message in the fault table?