1. I'm not sure, but there's an easy way to find out.
Locate and test the two starter wires. Also, locate and test the constant power wire.
Turn the key to the Run position and leave it there. Use a temporary jumper wire to touch the constant power wire to just one of the starter wires...and then just to the other starter wire.
You'll find that powering one of the starter wires all by itself will have no effect, and the other starter wire all by itself will cause the car to crank, but not start most of the time.
Cut the wire in half that made the starter crank. Now, try starting the car with the key.....will it crank? If not, great! Just do an anti-grind setup on this one wire, and you'll be fine. If the car still will crank, I guess you'll have to interrupt both wires.
2. The test mentioned by kickyride is an accurate one; you should perform it. But I can tell you from experience...the Maxima will disarm all by itself upon remote starting.
3. The 555U is simple as can be. You'll need a long-shanked Phillips screwdriver to get the steering column shroud screws out, and then the shroud comes apart like to clamshell halves. Toss the 555U ring around the key cylinder any way you like; Nissans aren't fussy.
Should you cut the shaft off the key? It's a good idea for security purposes, but remember, if you should screw anything up and have to visit the dealer for key re-programming, or if the keys should fail for some other reason that isn't even your fault...the dealer isn't going to know how do deal with the key you've cut. At the very least, don't cut the key until the rest of your installation is complete and working properly.
If you wanted to get really involved, you could remove the little rectangular plug from the key, and try to slide the transponder out. You could glue the transponder into the 555U, but save the rest of the key for future re-use.
Also, I believe transponderbypass.com makes a no-key-required bypass for Nissans; I've never seen or tried it though.
4. No. Here's how it's done. When testing the two so-called "accessory" wires, pay close attention---they do not operate the same way. One wire will only show power in the Run position; I like to call this heater-accessory or HVAC. The other wire will show power in the ACC and RUN positions; I like to call this radio-accessory.
Now is time for you to make a decision---do you want the radio to come on with the remote starter? Suppose you leave it set loud, and it disturbs your neighbors at 6AM when you start the car? On the other hand, some people think it's kinda cool to have tunes coming out of the car when you're parked at your favorite hangout spot.
But it is your decision; the radio-accessory wire is not needed for remote start. I've got about a half-dozen Maximas and a good 40-50 Infiniti I-35s done this way; customers are very happy with it.
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Also, how would you like some other inforamtion to help you along? I've done a fair amount of these cars, so here's what I've found.
A. The ignition wires are a bit hard to get, but much easier if you know the trick. Coming out the right of the steering column are two plastic tubes wrapped in gray tape; the lower tube contains the wires you want.
You'll also notice a huge hunk of metal just to the right of the tubes----I like to call this part the Fuse Blower. Anyway, step 1 is to lay duct tape over the Fuse Blower; trust me.
Then, cut the zip ties holding the tube in place, and pull it down a bit. Run your fingers around the tubing until you feel a slit under the tape; slice through here with a razor, and then pull the tube away. The tube will no longer fit here after your install, so leave the duct tape on the Fuse Blower for protection.
Also, not one of the ignition wire colors on DEI's tech sheet is correct. After you get the column shroud off, you can see six large wires coming out of the left side of the ignition switch. There's no room here to make splices, but you can make note of the colors, then find the same wires where you took the gray tube off. You'll have to test each of these wires for proper function, but that's not too hard---it sure beats having to test every wire in the harness!
B. Getting the tach wire right at the ECM plug is very difficult at best; there's a far better way. Pull out the black panel to the right of the gas pedal, and open up the large harness you see there. Dig deep into the harness; you'll eventually get the WHITE/ green tach wire, with plenty of slack for an easy connection. At one point, the wire branches off two ways, with a bit of blue tape on it---these clues will help you find it more easily.
C. Getting the brake wire at the switch is pretty much impossible. Remove the driver's kick panel, and get it in the large wire harness that runs to the rear of the car. You can also get positive parking lights here, but the 551T can't do positive parking lights without an extra relay. Easier to just get the negative lights.
D. The black metal knee bolster--the part you removed from below the steering column with two 10mm bolts--could also turn into a short-circuit hazard, as it will rest against the area where you made your ignition wiring connections. Put some type of insulating material between the two to prevent this. I use duct tape, but I'm sure you can think of something more elegant.
E. You're using a 551T, right? There's an easy way to make your second starter wire. Remember, you won't need to connect the unit's pink/white "ignition 2" wire to anything, so we'll "borrow" this for the second starter. See the little thin pink wire going into the ignition 2 relay? Cut it away, and splice it into the little purple wire----you've just turned the pink/white into a second starter output.
F. How are you going to trigger the 551T? You'll probably want to find the "lock motor" wire in the car, and invert it to a ground with a relay. The "lock motor" wire in the car isn't listed in TechSoft, and I can't remember the color. But it's at the SECU; the same place you catch hoodpin. It's a medium-size wire, and I think it's in the plug that's closer to the front of the car. Sorry I don't have better information----you'll just have to test all the suspect wires until you find it.
Also, I'd imagine you'll probably set it up so that pressing "lock" 3 times will make the remote starter come on. Every time you press Lock, the car's horn honks; this may annoy you. I think there's a way to deactivate the honking in the owner's manual.
G. The negative parking lights wire can be found at the SECU---same place you got hoodpin and lock motor. The color should be the same as what's listed in Techsoft.