Diagnostic Methodology & Standards

1. OBD-II Code Architecture (SAE J2012)

Standard OBD-II trouble codes consist of 5 alphanumeric characters:

• 1st Character (System Category): P (Powertrain: Engine, Transmission, Emissions), C (Chassis: ABS, ESP, Suspension, Steering), B (Body: Airbags, Climate, Lighting, Locks), U (Network: CAN-Bus & Inter-Module Communication).
• 2nd Character (Standard vs OEM): 0 (SAE Generic / Standardized across all makes), 1, 2, or 3 (Manufacturer-Specific).
• 3rd Character (Subsystem): Defines the exact mechanical or electrical subsystem (e.g. 1/2 = Fuel/Air, 3 = Ignition, 4 = Emissions, 7/8 = Transmission).

2. Severity & Safety Criteria

Fault codes are classified by mechanical risk: Minor (safe to drive, emissions-only), Moderate (safe to limp home, risk of elevated fuel consumption or long-term catalyst fouling), and Critical (misfire, overheating, or transmission slipping — vehicle must be stopped to avoid catastrophic damage).