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Can someone tell me the dimensions of the triple grille slots on their hood, please? Looking for: Width of a slot Height of a slot Total width across all 3 slots (i.e., outer-outer including metal gaps)

Howdy. If you're not familiar with the Factory Service Manual, they can be extremely helpful: https://www.nicoclub.com/nissan-service-manuals. Recommend to download each PDF for your model year (there's a download link below the inset PDF viewer). The General Information (GI) is a great starting point for how to read diagrams, wire colors, harnesses, etc. You'll want the Automatic Transmission (AT) chapter for the P0740: https://www.nicoclub.com/service-manual?fsm=Pathfinder%2F2002%2Fat.pdf You'll want the Engine Control (EC) chapter for the others: https://www.nicoclub.com/service-manual?fsm=Pathfinder%2F2002%2Fec.pdf For the P0740, ignore the "Starter Signal Circuit" part of what your code scanner is giving. The code is for the torque converter clutch signal. You won't notice anything different for gears 1-3, as this applies to the transmission being able to lock up the torque converter while in 4th gear after the transmission has warmed up. Either the valve's not functioning, or the circuit from the transmission control module (TCM) to the solenoid is open. Lowest hanging fruit is checking the harness under the truck on the passenger side; should be a small heat shield to remove and also checking one of the wires for ground continuity. The solenoid requires some work to access from underneath the transmission with the pan off, so taking it to a shop depends on much you're willing to take apart. P0420 and P0430 are the same codes, one for each side/bank of the engine. It's unlikely that sensors on both sides and/or catalytic converters have crapped out at the same time, but because you're throwing both codes, there's something in common between bank 1 (cylinders 1, 3, 5 on passenger side) and bank 2 (cylinders 2, 4, 6 on driver side). Possibly a leak somewhere along the intake (i.e., anywhere between the MAF, along the intake plenums, down to the fuel injector rail and intake manifold atop the engine), which changes the air metering the engine expects to burn. There are metal gaskets at every connection; maybe there's a leak. Could also be coil pack(s) related, but again, it's likely something in common...one bad coil pack might only trigger one of the codes, but not both. Last time I was chasing those codes (P0430), it was due to ill-fitting fuel injector o-rings when I serviced them. It was a frustrating process to figure it out, eventually discovered using a smoke machine to detect the leak. Lastly, the oil leak. Does seem likely valve covers. I think the gaskets tend to leak at the front corners of them, near the 1 and 2 cylinder coil packs. They do require RTV sealant in those spots and it wouldn't surprise me if it's time to re-seal those spots, particularly if it's not been done within those 277K. If you're planning to investigate the P0420/P0430 issues on the engines top end, it's a good excuse to tackle the valve covers when you get things pulled off anyway. The Engine Mechanical (EM) chapter is your friend here with all the torque specs and bolt tightening orders. https://www.nicoclub.com/service-manual?fsm=Pathfinder%2F2002%2Fem.pdf. Page EM-38 shows the points where RTV needs to be applied on each gasket. Overall job will take a few hours. Not a fun job, but also not too difficult.

Thanks so much @Slartibartfast, that should help out greatly. Will let you know how I go this weekend.

I started taking pictures, then realized that was dumb because it's tough to tell which way the camera's facing when it's inside the wheel well. So I took a short video instead. Hope it helps. (Dropbox link)