Piston rings and valves seal less tightly over hundreds of thousands of heat cycles. Combustion gas slips past instead of pushing the piston, so peak power softens. The NC's MZR engine is specifically known for the oil control rings sticking with carbon. Regular oil changes are the prevention; once damage is done, it's rebuild territory.

The timing chain stretches over time and the cam-timing system gets sluggish from old oil residue. The NC's peak power happens in a narrow band between 5,000 and 7,000 RPM, so even a few degrees of timing drift softens that whole upper band. Chain replacement and cleaning the cam-timing solenoid bring it back.

Spark plug electrodes wear down and the gap widens, so the coil packs have to work harder to fire each cylinder. Under high-RPM load there isn't enough time to build full spark voltage, and the engine misfires here and there. Each partial misfire is power gone. Fresh plugs and coils get it back.

Over 100,000 heat cycles the injector tips harden and varnish builds up, changing the fuel spray pattern. Each cylinder ends up with a slightly different air-fuel mix, and the engine's computer pulls timing on the lean ones. A fuel-pressure regulator that's stiffened can also sag rail pressure right when you need peak power. Injector cleaning or replacement fixes most of it.

Carbon builds up on the throttle body and the airflow sensor gets coated with oil mist over time. The sensor reads less air than the engine is actually getting, so the computer runs slightly lean across the board. A throttle body clean and an MAF sensor wipe are both cheap and noticeable.

The catalytic converter slowly breaks down inside, which raises backpressure. The engine's computer notices and pulls power on purpose to keep the cat from overheating. The oxygen sensors that report back to the computer also get lazy after 60,000 to 80,000 miles. A high-flow cat and fresh O2 sensors are one of the bigger recoverable wins on a high-mileage NC.

Bearings inside the engine wear smoother but lose film thickness. The oil pump wears, so peak oil pressure drops at high RPM. None of this is dramatic on its own, but it adds up to a small parasitic loss that you can't service away short of a rebuild. This is the floor the mileage deduction won't go below.