MCP2515 CAN module with Arduino Uno — read-only CAN bus sniffer

CAN is the bus that ECUs, dashboards and body modules use to talk to each other in cars, two-wheelers, solar inverters and industrial machines. An MCP2515 module with a TJA1050 transceiver lets an Uno listen in for about ₹200.

On the Uno it is an easy fit: the module is a 5 V part, so it connects directly to hardware SPI (D10–D13) with its INT line on D2. The sketch starts the controller at 500 kbit/s and immediately switches it to listen-only, so it logs every frame it sees without ever sending anything on the bus.

Use it on your own vehicle, a bench setup or equipment you own. Reading your own car’s bus is a great way to learn; sending frames into it is not.

Wiring

Board quirks

• Read-only, your own vehicle only. The sketch puts the MCP2515 in MCP_LISTENONLY mode: it never transmits, never acknowledges frames and so cannot disturb the car’s bus. Don’t change it to normal mode on a vehicle — a wrong frame can upset brakes, airbags or the engine ECU. Many newer cars put a gateway in front of the OBD-II port, so you may see nothing until a diagnostic request is sent; that is by design.
• Check the crystal. Most blue modules have an 8 MHz crystal (MCP_8MHZ). Some have 16 MHz. The wrong setting halves or doubles the bit rate and you receive nothing.
• Most cars run OBD-II CAN at 500 kbit/s; trucks and some ECUs at 250 kbit/s. Set BITRATE to match.
• Termination: the module’s J1 jumper adds a 120 Ω resistor. A car’s bus is already terminated — remove J1 there. On a two-module bench bus, fit it on both ends.
• Busy buses send thousands of frames a second. At 115200 baud the Uno can print only part of them; the sketch prints what it can and the MCP2515’s two receive buffers drop the rest. Filter by ID (init_Mask/init_Filt) to see one message cleanly.
• Power the Uno from the laptop’s USB, not from the car. Join the module GND to the car’s signal ground (OBD-II pin 5).

Code

// MCP2515 CAN module on an Arduino Uno R3 — LISTEN-ONLY sniffer for your own vehicle or test bench.
// VCC → 5V, GND → GND, CS → D10, SI → D11 (MOSI), SO → D12 (MISO), SCK → D13, INT → D2.
// Listen-only mode never sends a frame or an acknowledge, so it cannot disturb the bus.
#include <SPI.h>
#include <mcp_can.h>

const uint8_t CS_PIN = 10, INT_PIN = 2;
const uint8_t BITRATE = CAN_500KBPS;  // most cars' OBD-II CAN; try CAN_250KBPS for trucks / some ECUs
const uint8_t CRYSTAL = MCP_8MHZ;     // read the crystal can: 8.000 → MCP_8MHZ, 16.000 → MCP_16MHZ
MCP_CAN can(CS_PIN);
uint32_t frames = 0;

void setup() {
  Serial.begin(115200);  // CAN traffic is fast: 9600 baud would drop lines
  pinMode(INT_PIN, INPUT);
  if (can.begin(MCP_ANY, BITRATE, CRYSTAL) != CAN_OK) {
    Serial.println(F("MCP2515 init failed - check SPI wiring, 5V and the crystal setting"));
    while (true) delay(1000);
  }
  can.setMode(MCP_LISTENONLY);  // begin() leaves it in loopback; never switch to MCP_NORMAL on a car
  Serial.println(F("Listening (read-only)..."));
}

void loop() {
  if (digitalRead(INT_PIN) == HIGH) return;  // INT goes LOW when a frame is waiting
  unsigned long id;
  uint8_t len;
  uint8_t data[8];
  if (can.readMsgBuf(&id, &len, data) != CAN_OK) return;
  frames++;
  bool ext = id & 0x80000000UL;
  Serial.print(frames);
  Serial.print(ext ? F("  EXT 0x") : F("  STD 0x"));
  Serial.print(id & 0x1FFFFFFFUL, HEX);
  Serial.print(F("  ["));
  Serial.print(len);
  Serial.print(F("] "));
  for (uint8_t i = 0; i < len; i++) {
    if (data[i] < 0x10) Serial.print('0');
    Serial.print(data[i], HEX);
    Serial.print(' ');
  }
  Serial.println();
}

// MCP2515 CAN module on an Arduino Uno R3 — LISTEN-ONLY sniffer for your own vehicle or test bench.
// VCC → 5V, GND → GND, CS → D10, SI → D11 (MOSI), SO → D12 (MISO), SCK → D13, INT → D2.
// Listen-only mode never sends a frame or an acknowledge, so it cannot disturb the bus.
#include <SPI.h>
#include <mcp_can.h>

const uint8_t CS_PIN = 10, INT_PIN = 2;
const uint8_t BITRATE = CAN_500KBPS;  // most cars' OBD-II CAN; try CAN_250KBPS for trucks / some ECUs
const uint8_t CRYSTAL = MCP_8MHZ;     // read the crystal can: 8.000 → MCP_8MHZ, 16.000 → MCP_16MHZ
MCP_CAN can(CS_PIN);
uint32_t frames = 0;

void setup() {
  Serial.begin(115200);  // CAN traffic is fast: 9600 baud would drop lines
  pinMode(INT_PIN, INPUT);
  if (can.begin(MCP_ANY, BITRATE, CRYSTAL) != CAN_OK) {
    Serial.println(F("MCP2515 init failed - check SPI wiring, 5V and the crystal setting"));
    while (true) delay(1000);
  }
  can.setMode(MCP_LISTENONLY);  // begin() leaves it in loopback; never switch to MCP_NORMAL on a car
  Serial.println(F("Listening (read-only)..."));
}

void loop() {
  if (digitalRead(INT_PIN) == HIGH) return;  // INT goes LOW when a frame is waiting
  unsigned long id;
  uint8_t len;
  uint8_t data[8];
  if (can.readMsgBuf(&id, &len, data) != CAN_OK) return;
  frames++;
  bool ext = id & 0x80000000UL;
  Serial.print(frames);
  Serial.print(ext ? F("  EXT 0x") : F("  STD 0x"));
  Serial.print(id & 0x1FFFFFFFUL, HEX);
  Serial.print(F("  ["));
  Serial.print(len);
  Serial.print(F("] "));
  for (uint8_t i = 0; i < len; i++) {
    if (data[i] < 0x10) Serial.print('0');
    Serial.print(data[i], HEX);
    Serial.print(' ');
  }
  Serial.println();
}

Library: mcp_can (coryjfowler) — compiled with mcp_can 1.5.1.

Expected serial output

Listening (read-only)...
1  STD 0x316  [8] 05 1C 0C 1C 0C 00 00 00
2  STD 0x329  [8] 40 8A 7B 12 00 00 00 00
3  STD 0x18F  [8] FE 33 00 00 00 00 00 00

Listening (read-only)...
1  STD 0x316  [8] 05 1C 0C 1C 0C 00 00 00
2  STD 0x329  [8] 40 8A 7B 12 00 00 00 00
3  STD 0x18F  [8] FE 33 00 00 00 00 00 00

Common errors