2000 Nissan Maxima Can't read OBD-II Codes?

2000 Nissan Maxima.

My Service Engine Soon light came on. When I tried reading OBD-II codes with my Innova 3100 I just get Err, no codes, also can't reset. I removed the battery cable, checked all the fuses, replaced a couple (including lighter) just for good measure, replaced the batteries in the code reader. Still can't read codes. The reader worked fine on this same car 2 months ago and still works fine on my other car. The car seems to be running fine but I need a smog test and can't with the dang light on. Any ideas? Thanks.