Hi - according to memory suppliers your T23 has the following memory capacity / requirements:
- IBM ThinkPad T23 maximum RAM capacity: 1GB
- Your IBM has Two (2) memory slots or sockets
- Module upgrade PIN count is: 144-PIN
- The memory speed is: SDRAM 133MHz, PC133
- The laptop DIMM type for the ThinkPad T23 is: SODIMM, SO-DIMM, Small Outline DIMM
- Non ECC
If you are meeting these requirements, you may have a defective module.
Also, according to Lenovo's page
on the machine:Memory
system has 128MB or 256MB of memory standard and can be configured for
up to 1GB of addressable memory. Memory options up to 512MB SDRAM SO
DIMMs each can be added in the two available slots for memory expansion.
The standard 128MB of memory is expandable up to 640MB and the 256MB of
memory is expandable up to 768 MB of addressable memory with 64, 128,
256, or 512MB SO DIMMs without removing the 128MB SO DIMM that is
standard in one of the internal slots for memory. For customers that
need the maximum 1GB to which memory can be expanded, it is necessary to
replace the 128MB or 256MB card with a 512MB card plus add another
512MB card in the extra slot. Only 133MHz DIMM memory can be used in
this system. The use of 100MHz DIMM memory is not supported in the T23
This means yo can trouble shoot the 512MB modules individually by installing one of the original 128MB modules and one of the new 512MB modules (for a total system memory of 640MB). If it does not boot, try again with the other 512MB module. This will help you identify a defective module that may be preventing the machine from booting.
I hope this helps & good luck!