ISO 9141-2:1994

Overview

ISO 9141-2:1994 - "Road vehicles - Diagnostic systems - Part 2: CARB requirements for interchange of digital information" defines a CARB-focused subset of ISO 9141 for on‑board diagnostic communication between vehicle ECUs and an SAE OBD II scan tool. It is limited to nominal 12 V vehicle systems and was published to facilitate compliance with California Code of Regulation, Title 13, §1968.1 (malfunction and diagnostic systems requirements). The standard specifies initialization, header formats, signal levels, timing, and electrical limits needed for reliable exchange of emission‑related diagnostic data.

Key topics and technical requirements

• Scope & compatibility
  • Subset of ISO 9141:1989 targeted at OBD II communication with SAE J1978 scan tools.
  • Connector compatibility per SAE J1962.
• Physical lines
  • Supports either one‑wire (K line) or two‑wire (K and L lines) configurations.
  • K line: bidirectional after initialization; used for request/response messages.
  • L line: unidirectional during initialization only; otherwise idles at logic “1”.
• Initialization & header
  • Diagnostic tester transmits a 5‑bit/s address (standard address 0x33) to invoke the protocol.
  • Initialization includes speed synchronization, two key words, logic inversions for handshaking and configurable P2 response intervals.
• Signal & timing
  • Logic thresholds: transmitter logic “0” < 20% VB, logic “1” > 80% VB (receivers slightly different: 30%/70% VB).
  • Bit coding: NRZ, slope time limits and sampling tolerances defined.
  • Communication baud after init: 10.4 kbit/s ±1%.
  • Key timing windows W0–W4 and P2 intervals specified (see Annex A in the standard).
• Electrical limits
  • Diagnostic tester cable capacitance CTE ≤ 2 nF; vehicle line capacitance Cosw ≤ 2 nF; total bus CEcu + Cobw < 7.6 nF.
  • Tester pull‑up: nominal 510 Ω ±5% to VB; operation over VB = 8–16 V and temperature 0–50 °C.
• Protocol
  • Subsequent message protocol aligns with SAE J1979 (with modifications noted in clause 11); typical OBD II messages are 5–11 bytes.
• Safety & robustness
  • Error handling, message timing, and worked examples are included to aid correct implementation.

Applications and users

• Automotive OEMs and system architects designing OBD II‑compliant ECUs
• ECU suppliers and embedded firmware developers implementing K/L‑line diagnostics
• Diagnostic tool and scan‑tool manufacturers (SAE J1978 compliance)
• Regulatory and compliance engineers ensuring California (CARB) OBD II conformance
• Aftermarket service and repair professionals needing to understand low‑level communication behavior

Related standards

• ISO 9141:1989 (Part 1 - full interchange requirements)
• SAE J1978 (OBD II scan tool)
• SAE J1962 (diagnostic connector)
• SAE J1979 (diagnostic message formats)
• California Code of Regulation, Title 13, §1968.1

Keywords: ISO 9141-2:1994, OBD II, CARB, diagnostic systems, SAE J1978, K‑line, L‑line, SAE J1962, ECU communication, vehicle diagnostics.