On my 02 model, the sensors are about 6 inches apart from left to right and only about 12" apart from fronts to rears. Getting bank 1 and 2 mixed up would be quite easy on my wagon.

 

You should have 4 wires for all the sensors (if you are running the post cat sensors). 2 on the passenger side, 2 on the driver side coming from the engine bay. One of each side is very long for the rear 02's. The front O2 sensors should have about 18" lead on it. They should reach to the ground.
With Long Tube headers, the sensors are just inches from each other. With OEM manifolds, they are feet apart.

 

Verano, yeah: on my 2000 the front sensors are just below the header flange, and they are quite a ways apart. I can't imagine trying to pull the left wire all the way over to mate up to the right sensor.
You'd have to rip the connectors off the firewall to make it reach.
I hear this happens often, so I guess the 2001-up have the front sensors located farther down the pipe, where they would come closer together.

As to the rear sensors, yeah, very easy to mix the wires, as they are installed criss-crossed from the factory. But, as stated above, the rears only roughly monitor the O2 content just to be sure the cats are doing something, and they can be unhooked or deleted permanently and not cause any running issues at all; they just set an information code that says your cat may not be working right.

DG

 

Ok, so I'm not crazy.

 

The issue is (at least with LG Longtubes) the headers come with harness adaptors to allow you to use the rear sensors in the front position. These adaptors are there because the rear sensors and front sensors use differently shaped connectors. The reason you need the rear sensors in the front position is because the long tube headers move the sensor's location farther away from the origional connector (new location is the collectors near the base of the bell housing). Unless you build your own harness extension the stock front sensors will no longer reach to the connectors mounted to the frame rails.

The mistake in crossing the two wires is made (at least when I messed it up) because when you use the supplied adaptors and rear sensors you end up with quite a bit of excess wire. As the rear sensors crossed it makes logical sense to do the same at the front. The result is the incorrect fuel trims and a misfire/surging feeling at part throttle (VERY noticible). At full throttle you will not notice any problems.

Clear as mud? As sad as this sounds... and as damaging to my confidence as a gearhead... it took two calls to LG Motorsports and an hour or so of pulling codes, staring at the wiring, and scratching my head to figure that out. In all honesty the first call was me asking what the sensor adaptors were for and the second was me asking why my car was running like @#$%. The very patient person on the other side calmly and simply asked, "Are the sensors crossed?" I felt like I should hang up my tools in embarrasment...

 

Ah. Now that makes sense. Now I can understand how it happens.
Having not put headers on my car (yet) I always wondered what the O2 mix-up was all about.

Thanks you.

DG

 

yep... once I messed it up I understood pretty well too. :o Hope my mistakes help someone else.

 

Yes, its only really for LT headers. I guess different brands have varying degree's of potential. But, definately not an issue with the stock manifolds. IMO I never call the stock manifolds "headers", to me headers are aftermarket.