The CN3 port is a 5-pin serial communication header found on most OBD0 Honda ECUs.

Functionality#

This port is functionally equivalent to the CN2 serial port found on later OBD1 Honda ECUs. It is primarily used for:

• Data Logging: Communicating real-time sensor data from the ECU to a PC-based tuning interface.
• ROM Modification: Facilitating the reflashing or communication required for external EPROM tuning boards.

Pinout Mapping#

The pinout for the CN3 header largely mirrors the OBD1 CN2 standard. When wiring a data logging cable (e.g., using a USB-to-TTL adapter), ensure the transmit (TX) and receive (RX) lines are mapped correctly according to standard ECU communication pin-outs.

Always verify your specific ECU board's silkscreen labels if available, as variations can occasionally occur between different ECU part numbers.