Hi, have headers? You can waste your money on o2 extenders or use common sense and use the following options for easy plug and play extended o2 sensors.

A32 Chassis
-Upstream
95-99 (96-99 I30) Federal Emissions - Not needed. The factory 28in wire length will get the job done.
99 Cali Spec - Use the non cali spec sensor for the front bank, or the factory rear sensor for the front.

-Downstream
95-99 (96-99 I30) Federal Emissions - Not Needed. It's after (or inside) the 3rd cat.
99 California - [Front Downstream] ****** I have never came across this before, someone will have to chime in here, but i doubt the factory length will work on OBX headers secondary ports ********

A33 and L31 Chassis
-Upstream
00 Federal Spec - You're good to go bud.
00-01 Cali Spec (ALL 2001 MAXIMAS ARE "CALI" SPEC) - Use the non cali spec sensor for the front bank, or the factory rear sensor for the front.
02-03 Maxima (02-04 Altima 3.5's) - Bosch 15363 or equivalent

-Downstream
00 Federal Spec - Nothing special needed
00-01 Cali Spec - Same problem as the a32 chassis, someone will have to chime in on their personal experience.
02-03 Maxima - Nothing special needed
02-06 Altima - Green BOSCH 15224 / Blue BOSCH 15225 (These will reach the cheap $150 ebay headers!)

A34 and L31 Chassis
-Upstream
04-08 Maxima (05-06 Altima 3.5s) - BOSCH 17179

-Downstream
04-08 Maxima (02-06 Altimas as well in case you missed the section above) - Green BOSCH 15224 / Blue BOSCH 15225 (These will reach the cheap $150 ebay headers!)

 

The 99 I parted out had 2 upstream sensors and 2 downstream sensors with a similar precat system as an 02-03 A33.
I might still have the sensors kicking around I could measure and take photos of.

 

It says it's approx 18 inches, but the real question is if those sensors can reach the secondary ports on popular headers..... most likely you'll have to weld an extra bung nearby for those cali specs.... That extra sensor has a unique plug, you can modify a sensor from other nissans of the time period to fit of course but again... I have 0 experience w/ cali spec vehicles.

 

On my 99 I had no issues running the stock o2 sensors in the headers. I did have to wire the 2 down stream sensors together. I am running a header back cattmen set up.

 

Good to know, if you have 2 downstreams you are a Cali-Spec with Swirl Valves and 4 total o2 sensors. Can you detail what you did so I can update my OP?