The ABS problem is not such a big deal (in terms of passing the smog check, might be a huge deal for driver safety), the real concern is the vehicle communicating with the smog inspection machine... or more accurately, the vehicle not relaying emission data to the test equipment via the data link connector (DLC). The reason this may be happening may very well be related to the ABS fault, and we recommend having a smog check repair station inspect both issues. Perhaps your Xterra has a power or ground problem which is cutting communication of both the ABS and DLC systems. Only a hands on diagnosis will tell.
On a side note, in case we've misunderstood the matter, and the actual problem is that the ABS system is defective and the vehicle simply has incomplete emission monitors, please follow the link below to learn how to make your Xterra's emission monitors ready for the smog test. We've had vehicle owners in the past mention their smog technicians have said the vehicle has no communication with the smog machine, when in fact what the technicians should have said was that the vehicle has incomplete monitors. These two problems are really worlds apart, but sometimes miss communication does occur and one problem gets confused for the other. So, in case we are in that situation here, please follow the link below. If in fact the only problem with your Nissan Xterra is that the emission monitors are incomplete, if you can get the monitors to become "ready" or "complete", the ABS problem will more than likely not cause a smog check failure. The California smog inspection does not test a vehicle's ABS system.
How to Complete Nissan Xterra Drive Cycle
