For some models you can use default immobilizer code - "5523". Such vehicles have a big letter "A" on ICU label. Following vehicle models have a default pin code:
Nissan Altima 2001-2004;
Nissan Maxima 2001-2003;
Nissan Pathfinder 2001-2004;
Nissan Sentra 2000-2005;
Nissan 350Z < 2002;
Nissan X-Terra 2003-2004;
Infinity QX4 2001-2004;
Infinity QX45 2002;
Infinity Q45 2001-2005;
Infinity G20 2000-2002;
Infinity I30 / I35 2001-2005.
To identify NATS version - check label on ICU: STEP-5 means NATS-5, STEP-5.6 means NATS-5.6, STEP-6 means NATS-6 and STEP-7 means NATS-7.
For Primera P12, new Almera and some other models equipped with NATS-6 immobilizer pin code must be calculated by serial number from SEC label. For models with NATS-5 immobilizer - by serial number from BCM or ICU label. For models with NATS-7 (based on Renault-Samsung platforms) by dump from BCM.
NATS also known as NVIS - Nissan Vehicle Immobilizer System and IVIS - Infiniti Vehicle Immobilizer System.