Discovery 3 CAN Bus Sniffing, Part 3: When OBD-II Frames Stay at Zero

During the Discovery 3 CAN sniffing setup, the LilyGO board was working over USB and could transmit a diagnostic request, but the frame counter stayed at zero. This is the checklist that came out of that session.

1. Confirm the adapter is really present

On Windows, the LilyGO board should show up as a USB serial device, not just the machine internal COM port.

Good: COM23 = USB-Enhanced-SERIAL CH9102
Ignore: COM3 = Intel AMT internal serial

If the board does not appear, try another USB-C cable before changing the vehicle wiring. In this case, a bad LISEN cable was enough to block progress completely.

2. Start in listen-only mode

Listen-only mode is the safest first test because it does not transmit onto the vehicle bus.

mode listen
speed 500
start
status

For the tested firmware, two LilyGO pin mappings were worth checking:

pins 5 4
pins 27 26

3. If passive capture is quiet, send one known diagnostic request

A standard OBD-II RPM request is a useful sanity check:

mode normal
speed 500
start
obd 0C

The transmitted frame is:

ID 0x7DF: 02 01 0C 00 00 00 00 00

If transmit reports OK but there are still no replies, do not assume the vehicle is silent. Check the physical mapping.

4. Measure by pin, not by wire colour

At the OBD port itself, measure CAN-H and CAN-L against signal ground:

Black probe: OBD pin 5
Red probe:   OBD pin 6  - CAN-H, usually about 2.5 V when awake
Red probe:   OBD pin 14 - CAN-L, usually about 2.5 V when awake

Measuring between CAN-H and CAN-L with a multimeter can be misleading because both lines idle near the same common-mode voltage.

5. Continuity map the pigtail

The pigtail used in this test did not match its supplied colour chart. The actual mapping was:

OBD pin 6  = Green       = CAN-H
OBD pin 14 = Green/White = CAN-L
OBD pin 5  = Light Blue  = Ground

That means the correct LilyGO wiring is:

Green       -> CAN-H
Green/White -> CAN-L
Light Blue  -> GND

6. Treat OBD pin 16 carefully

OBD pin 16 is battery positive. On a DIY pigtail, identify it by continuity before using or insulating it. Do not trust a seller colour chart where the cable has already been proven wrong.

The practical order is: USB detection, serial firmware, OBD port voltage, cable continuity, then CAN capture. Once the physical mapping is correct, the software tests become meaningful.