First digit — the system

• Pxxxx — powertrain
• Bxxxx — body
• Cxxxx — chassis
• Uxxxx — future systems

Second digit — who defined it

• P0xxx — Government required codes
• P1xxx — Manufacturer codes for additional emission system function; not required but reported to the government

Third digit — the subsystem

• Px1xx — measurement of air and fuel
• Px2xx — measurement of air and fuel
• Px3xx — ignition system
• Px4xx — additional emission control
• Px5xx — speed and idle regulation
• Px6xx — computer and output signals
• Px7xx — transmission
• Px8xx — transmission
• Px9xx — control modules, input and output signals

The fourth and fifth digits designate the individual components and systems.