'92-'01 Honda OBD Data Reader

Table of Contents

• '92-'01 Honda OBD Data Reader
  • Table of Contents
  • Introduction
  • Data
  • Requirements
  • Installation

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.

     

   • 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.

     

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

   • SoftwareSerialWithHalfDuplex.

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

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