Heineken's Garage

Introduction
Failure of the Climate Control Unit (CCU) is a common fault, affecting nearly any NSX. In almost any case, capacitor replacement and track repair will fix all issues. Depending on the state of the PCB, further damage to other components can happen, though.

Diagnostic - Transistor Issues
The following error cases and ways of repairing have been identified so far. These faults can be caused by major acid leakage affecting nearby transistors.

Interior fan working only in automatic mode
If that's the case, regardless of the dial setting (don't mistake it with a CCU that works only on the highest dial setting) it can be related to a faulty Q17. The transistors Q14 to Q17 are level shifters providing the position of the fan speed dial to the large Microcontroller.
They are NPN transistors with an internal 4.7 kΩ base resistor (NEC A1L3Z). Such parts are no longer available as a direct replacement but still sold in SOT 323 SMD packaging (ADTC143TUAQ) - which requires appropriate adaptation work (SOT23 adapter PCBs):

AC Compressor and Fans not coming on
.. even though they should (AC isn't OFF) can be caused by a faulty Q9. This transistor (NEC AN1L3N) is a PNP type with two integrated resistors: 4.7 kΩ (R1) at the base and 10 kΩ (R2) from base to emitter. Same as with Q14 to Q17 there are no drop-in-replacements available any more. A possible SMD replacement is PDTA143X with corresponding adapter PCBs:

Q9 drives Q11 and Q11 drives the final output signal for the AC. Being very close to a capacitor it's sometimes damaged up to the point that its feet are completely corroded off. It's a 2SD1779, a very special type of transistor. It can be replaced by a BPS295 MOS-FET and requires an adapter board:

Q12, that controls the blower fan relay, can be replaced with the same type.

Diagnostic - Specific PCB Tracks
These error cases are related to specific broken tracks on the PCB that are hard to detect and/or who's resulting issues are fully understood.

Full functionality but display stays off
If the CCU shows full functionality but the display and LEDs stay off, it could be due to this broken trace near C44:

CPU stays in reset (no function of the CCU)
A missing connection from the positive pin of C14 to D7 and D6 keeps the processor in a reset loop. This trace damage makes the analog input pins 60, 61 and 62 of the CPU drop to near zero (should be 2.4 V) causing a reset, as soon as reaching below 1 V.
Temporary lifting the analog ground connection pins of the CPU (pin-pair 52/54) from ground should enable the processor again. The voltage at the lifted pins should be ~4.88 V. If it's significantly less (e. g. 2.7 V) the CPU might be damaged.

Interior fan working only on highest setting
The classical issue observed due to capacitor damage to the CCU. If the negative pin of C19 isn't connected to ground any more, it lifts the CPU pin pair 52/54 (CPU analog ground) and causes the processor pin 28 to stop providing a PWM signal for the fan control.

Display stays off, weak beep
If the processor is confirmed to be running (see below) but the display stays off and the confirmation beep is only faint (sometimes completely failing after the first press) the 5 V reference value for the sensor network should be checked. The reference voltage can be measured at pin 12 of the CPU (and others). It runs all around on the outside of the PCB's underside as well and is even found in the small green connector for the vehicle loom.

A too low voltage prevents the sensors from working as well as the beep indicator and the display.
A broken trace near Q9 can cause such a drop in the reference voltage:

.. continues on Climate Control Unit (CCU) - Repair II