Towncivilian

Bottle of what?

Useless

I have not heard of the clogged catalytic converter problem. Power valve screws are a must to apply threadlocker to (preferably red as you shouldn't have to remove these again for any reason) when you are replacing your PCV valve. I would also install a PCV catch can while you're there to collect blow-by vapors and keep the intake cleaner. This guide on NICO is a DIY guide on reaching the PCV valve, and this post shows an example PCV catch can set up. Automatic VQ35DEs have power valves in the lower intake manifold which actuate by a vacuum. They open when a certain amount of throttle is pressed. As long as you match ECUs (i.e. use an auto ECU with an auto engine) you will not have any problems.

R50s have either 4.363 or 4.636 gearing, seemingly at random. My 2WD with tow package has 4.363.

R50s have separate TCMs.

Have you ever replaced the fuel filter?

Actually I think the transfer case may take GL-4 fluid only, Amsoil's vehicle lookup only lists GL-4. I tried to check the 89 D21/WD21 FSM but I couldn't find a recommended/specified lubricant chart, nor can I find the specified fluid in the "changing transfer fluid" section of MA. It's not listed in the "transfer" section either!

It's an auto, gear lube would be a bad idea to put in it.

Stick with the OE cap. See this thread on BITOG about various radiator brands.

Be sure it's been inspected for the strut housing corrosion recall.

Ever changed the transfer case fluid and transmission fluid? What's the trans fluid level, color, and smell?

Check the brake fluid level, low fluid can trigger the light. Matter of fact just flush your brake fluid. I would change the transmission fluid and add an auxiliary cooler bypassing the stock cooler, or plumb it inline with the stock cooler only after cleaning it out. An inline filter is also a good idea. Do a search, you'll find all the necessary info and fluid exchange procedures. I'm on my phone so excuse the brevity. Welcome to NPORA!

AutoGroan has Techron BOGO free, so get a couple 20oz bottles.

I would put a 20oz bottle in right before you fill up (87 octane only of course) and avoid long trips, but nothing so short that nothing heats up. Basically, drive as you normally would. If results are unsatisfactory, you can try with another 20oz bottle. AAP may have buy one get one for $1 this month, they did last month. WalMart also had $2 hang tag coupons (that ironically didn't even ring up in their store) but I peeled a few off and used them at AAP to get 2 20oz. bottles for $13 or so.

750 +/- 50

It doesn't matter which bottle you use, you can't overdose on Techron (PEA) in sane amounts (i.e. don't put three gallons of the stuff in your tank). It is best to do short trips to allow the cleaner to soak in, don't burn through the entire tank as you would on a lengthy highway trip for example.

Bypassing the amps is possible by either hacking up the wiring, or running your own speaker wiring. Running your own speaker wiring to each door is the better option, even though it requires a little more effort.

I would not use any cleaner/flush products in the cooling system. A drain & fill of the radiator and overflow reservoir will be sufficient. Use distilled or demineralized water only for the service. The brake bleed order is: 1. LSV air bleeder (Models equipped with LSV) 2. Left rear brake 3. Right rear brake 4. Left front brake 5. Right front brake 6. R-ABS actuator (Models equipped with R-ABS) I don't think any WD21 has an LSV (load sensing valve) so ignore that one. Use any GL-4 fluid for the manual trans. Amsoil MTG or Redline MT-90 are good choices. Open the fill plug first, otherwise if you can't get it loose you ain't going nowhere. Is your Pathfinder 4WD? If so, you'll need to overfill the trans; see this thread. Done the power steering fluid, differential(s), fuel filter, engine air filter (if it looks really dirty)? Got slack in the accelerator pedal? Adjust it according to FE-2 and also see EL-63 for adjusting the cruise wire.

Define "checked out". Fluid that dark has probably been in there for a while, and if the fluid contains just 3% water it's defined as "wet" and the boiling point is reduced by a hundred degrees or so.

As the pads wear the fluid level in the master cylinder lowers as the caliper pistons extend further to compensate. Low brake fluid can trigger the brake warning lamp, but when you top off the light goes off unless there is another issue.

Did you check the fluid level properly? 1. Drive around for 10 minutes in the city after the transmission is at operating temperature. 2. Park on a level surface and apply the parking brake. 3. With P selected, shift through every gear and return to P. 4. Pull the transmission dipstick with the engine idling, wipe it clean, reinsert it completely, and read the level. It must be within the notched area labeled "HOT". Check for fluid contamination after wiping on a paper towel. The fluid should be red, and not smell burnt. Sorry, I don't know the answer to your RPM question since my Pathfinder has the VQ35DE vs. your VG33E.

No, you can still pull codes regardless. There may be a pending or stored code. See this thread for instructions. The in-pan strainer is more of a rock catcher, it doesn't filter much. If that gets clogged, you have bigger problems. Drain & fills are more wasteful than a full fluid exchange. I would just do a cooler line exchange; if that doesn't help things, then do a pan drop to inspect and clean the pan magnet and swap out the strainer.

Your brake fluid looks dark, think about flushing the fluid soonish.

You're welcome, good luck! Hope it helps with the shifting issues.

If you don't know when the fluid was last changed, I would perform a full fluid exchange using the transmission cooler return line. You will need: 12 to 15 quarts of automatic transmission fluid, preferably Nissan Matic D; a fluid claiming to meet Dexron III is suitable as well. A clear gallon jug, something like an empty milk jug Another large container to store used fluid for disposal 1. Disconnect the transmission cooler return line and secure it in a collection bucket such as a clear gallon milk jug. This will be the hose on the passenger side of the vehicle; disconnect the end leading towards the transmission. 2. Have a helper start the car while you watch the bucket - once it's half full (2 quarts), yell for the helper to stop the engine. 3. Add 2 quarts of ATF through the transmission dipstick. 4. Drain collection jug into larger jug (i.e. a Disposoil - these only hold 10 quarts though in my experience, so have something else to hold the last 5 quarts). 5. Repeat steps 2-4 until you're out of ATF. 6. Check fluid for proper level, and recycle the old ATF. Any place that takes used oil will likely recycle ATF.