HP 3458A , resistance range out of order

[Kleinstein]

As Z300 was bad, there is chance that Q304 (the MOSFET used as a low leakage diode near Z300) is some what damaged and might show too much leakage for the lowest current range to work. For a first test one could simply add an extra low leakage diode in series at the Z300 pin.

Another possible source of trouble is leakage due to surface contamination.

So how much current do you get when the 100 nA source (100MOhms range ?) is selected ?
If possible measure at low voltage and something like 5 V.

Just in case, it might also be good to check input bias for the voltage ranges.