CAN Bus Basics — CAN-H, CAN-L and the 60 Ω Test
All CarsGeneral GuidesAll yearsData Lines (CAN)Updated 2026-10-10Modern cars have many computers (engine, ABS, meter, body…). They share information on two twisted wires called CAN-H and CAN-L. At each end of the bus there is a 120 Ω resistor. Many 'lost communication' (U-codes) faults are simply a broken wire, a bad connector or a missing resistor.
Pinout — pin number & wire colour
Test point: OBD2 port pins 6 (CAN-H) and 14 (CAN-L)
| Pin | Name | What it does |
|---|---|---|
| 6 | CAN-H | CAN high — about 2.5–3.5 V when data is sent. |
| 14 | CAN-L | CAN low — about 1.5–2.5 V when data is sent. |
CAN bus layout
- Each end of the bus has a 120 Ω resistor (usually inside two of the modules).
- Every module connects to both wires: CAN-H and CAN-L.
- You can reach the bus at the OBD2 port: pin 6 and pin 14.
- Battery disconnected, measure pin 6 to 14: about 60 Ω = OK. About 120 Ω = one end open. Near 0 Ω = wires shorted together.
🧰 Tools you need
- Digital multimeter
- OBD2 scanner (ELM327 or better)
- 12 V test light
- Back-probe pins / jumper wires
Common problems & quick checks
| Problem | Likely cause | What to check |
|---|---|---|
| Reads about 60 Ω | Both resistors and wires OK | Look for a module with no power or ground instead. |
| Reads about 120 Ω | One end of the bus is open (broken wire, unplugged module) | Unplug modules one by one to find which side is open. |
| Reads near 0 Ω | CAN-H and CAN-L shorted together | Check for crushed wiring and water in connectors. |
| CAN-H or CAN-L at 0 V or 12 V | Wire shorted to ground or to battery | Key ON, measure each wire to ground. Normal is about 2–3 V. |
Related trouble codes
U0001 — High Speed CAN Communication Bus
A module found a problem on the high-speed CAN data wires.
NetworkhighU0073 — Control Module Communication Bus "A" Off
A module stopped talking because the data bus is shut down or faulty.
NetworkhighU0100 — Lost Communication With ECM/PCM "A"
Other computers can no longer hear the engine computer (ECM).
NetworkmediumU0121 — Lost Communication With ABS Control Module
The ABS computer stopped sending messages on the data wires.
NetworkmediumU0140 — Lost Communication With Body Control Module
The body computer (lights, locks, wipers, chimes) is not talking on the data wires.
NetworkmediumU0155 — Lost Communication With Instrument Panel Cluster
Another computer in the car stopped getting messages from the meter. The meter has no power, no ground, or is not connected to the data wires.