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'92-'01 Honda OBD Data Reader

Table of Contents

Introduction

This sketch demonstrates how to communicate with the Honda 3-pin diagnostic connector, commonly found in Honda vehicles manufactured between 1992 and 2001, to extract real-time data from the ECU.

Data

The code reads and processes a variety of engine parameters. Below is a description of each variable and its significance and unit of measurement, followed by a table detailing the protocol commands and addresses tested on my '98 Civic with a D14A3 engine.

  • rpm: Engine revolutions per minute
  • speed: Vehicle speed (Km/h)
  • iat: Intake Air Temperature (°C)
  • ect: Engine Coolant Temperature (°C)
  • tps: Throttle Position Sensor (%)
  • o2: Oxygen Sensor reading (raw voltage in 0–255)
  • stf: Short Term Fuel Trim (%)
  • ltf: Long Term Fuel Trim (%)
  • ta: Timing Advance (°)
  • voltage: ECU Voltage (V)
  • imap: Intake Manifold Absolute Pressure (kPa)

Below is a reference table for each data request:

Parameter Command Address Bytes to Read Description
RPM 0x20,0x05 0x00 0x10 Engine RPM (2-byte data). Formula: ((dlcdata[2] * 256) + dlcdata[3]) / 4.
Engine Coolant Temperature (ECT) 0x20,0x05 0x10 0x01 Computed via a polynomial approximation, then offset by +40 to convert to °C.
ECU Voltage 0x20,0x05 0x17 0x01 Raw byte scaled by 1/10.45 to determine voltage (in volts).
Short Term Fuel Trim (STF) 0x20,0x05 0x20 0x01 Percentage indicating short-term fuel correction.
Long Term Fuel Trim (LTF) 0x20,0x05 0x22 0x01 Percentage indicating long-term fuel correction.
Intake Manifold Absolute Pressure (IMAP) 0x20,0x05 0x12 0x01 Raw byte scaled by 0.716, offset by -5 to convert to kPa.
Vehicle Speed 0x20,0x05 0x02 0x01 Vehicle speed in km/h (assumed 1:1 from raw byte).
Intake Air Temperature (IAT) 0x20,0x05 0x11 0x01 Similar polynomial to ECT, then offset by +40 to convert to °C.
Timing Advance (TA) 0x20,0x05 0x26 0x01 Formula: ((dlcdata[2] - 24) / 2) + 128. Represents degrees of ignition advance.
Throttle Position Sensor (TPS) 0x20,0x05 0x14 0x01 0% = 25 raw value, 100% = 233 raw value. Computed by (dlcdata[2] - 25) / 2.08, clamped to non-negative.
Oxygen Sensor (O2) 0x20,0x05 0x15 0x01 O2 sensor reading in a 0–255 scale (approx. 0–1 V or 0–5 V, depending on the ECU).

Requirements

  1. Honda 3-Pin OBD Port

    • The 3-pin DLC diagnostics data connector (blue; older versions ‘92–‘95 may be gray or yellow) is usually inserted in a protective rubber (green or blue) sleeve together with a 2-pin SCS connector. Both connectors might appear together in one protective sleeve.

      Honda 3-pin OBD connector

    • The 2-pin connector is a SCS service check connector for self-diagnostics, not for data transmission.

    • You must locate the standard Honda 3-pin diagnostic connector in your vehicle, which is often found near the passenger-side kick panel or underneath the dashboard.

      Honda 3-pin OBD connector locations

  2. Arduino Board

    • This code has been tested with a typical Arduino UNO. Make sure you have a free digital pin to handle half-duplex communication (pin 11 in this code).

  3. Libraries

  4. Connections/Wiring

    • Pin 11 on the Arduino is used for both RX and TX (half-duplex).
    • Proper grounding between the ECU and Arduino is critical.
    • A level shifter or special circuitry may be required, depending on ECU voltage levels.

Installation

  1. Library Setup

    • Install the SoftwareSerialWithHalfDuplex library. You can place the library folder inside your Arduino libraries/ directory or install via Arduino Library Manager if available.

  2. Open the Sketch

    • Copy the provided code into a new Arduino sketch file or clone your repository containing this code.

  3. Adjust Pin Assignments (If Needed)

    • By default, pin 11 is used for both RX and TX with half-duplex mode.
    • Ensure the pin is free and not used by other peripherals in your Arduino setup.

  4. Compile and Upload

    • Connect your Arduino to the computer via USB.
    • Select the correct Arduino board and COM port in the Arduino IDE.
    • Click Verify then Upload.

  5. View Data

    • Open the Arduino Serial Monitor at 115200 baud.
    • You should see CSV-like output printing the values for RPM, ECT, Voltage, etc.
    • If everything is connected properly, the parameters will update in real time.
    • A graphical user interface (GUI) is planned for future development to provide a more user-friendly experience. <3

Enjoy monitoring your Honda data! Feel free to modify the code for other parameters or to adjust the formulas to match different sensor curves.