What does the CAN bus troubleshooter do?
A CAN bus is a pair of wires, CAN-H and CAN-L, with a 120 Ω terminator at each end. Most "CAN not reading" problems are in that pair: a missing terminator, a wrong pair, a short, or a line pulled to ground or to the battery. A multimeter finds all of them in a few minutes.
This tool takes your meter readings and gives a finding for each one, with what it means and what to do. The Readings tab judges the resistance and the two voltages. The Bus length tab checks the main line against the bit rate.
Why a healthy bus reads 60 Ω
The two terminators sit at opposite ends of the pair, so from any point on the bus your meter sees them in parallel. Two 120 Ω resistors in parallel give 120 / 2 = 60 Ω. With one terminator you read 120 Ω. With three you read 40 Ω.
That is why the reading tells you how many terminators the meter can see. It does not tell you where they are. Measure from a point you can reach, then look at both ends of the bus.
What the voltages tell you
With the ignition on, both lines of an idle bus sit near the same level to ground. A DC meter averages the fast switching, so you read a steady value. The tool uses an idle window of 2.0 to 3.0 V (typical).
- A line near 0 V is shorted to ground, open or not driven.
- A line above 5.5 V is touching a supply wire.
- CAN-H well above CAN-L (1.5 V or more) means the bus is held dominant.
- CAN-L above CAN-H may mean the two wires are swapped. Treat it as an indication.
Typical limits. The idle window, the ground and supply levels and the tolerance are editable in the page configuration.
Length against bit rate
A longer bus needs a lower bit rate. The common table gives 40 m at 1 Mbit/s, 100 m at 500 kbit/s, 200 m at 250 kbit/s, 500 m at 100 kbit/s and 1,000 m at 50 kbit/s. The rule of thumb behind it is rate in Mbit/s times length in meters, 50 or less.
Vehicle networks that follow SAE J1939 can set tighter limits than this general table. Check the J1939 document or the vehicle maker for your bus.
Worked example
- Resistance: 118 Ω with the battery disconnected. 120 / 118 = 1.02, so the meter sees one terminator. The finding is a fault: one is missing or the bus is split.
- Voltages: CAN-H 2.6 V and CAN-L 2.4 V. Both are inside the 2.0 to 3.0 V idle window, 0.2 V apart. The finding is normal.
- Length: 60 m at 500 kbit/s. The limit is 100 m, so you use 60 % of it. The finding is acceptable.
- Result: fault found. Look for the second terminator before you touch any setting.
Reading the result
- Fault found means at least one reading points to a wiring problem. Fix it first.
- Check needed means no hard fault, but a reading is off. Measure again.
- Looks healthy means the wiring readings are normal. It does not prove the data is right.
- A reading taken in the wrong state is flagged and not judged.
Common mistakes
- Measuring resistance on a live bus. The modules change the reading. Power off first.
- Probing the wrong pins. A good reading on the wrong pair is a different circuit.
- Adding a terminator to fix a 120 Ω reading before you know where both ends are. You can end up with three.
- Reading voltage with the ignition off. The bus sleeps and the numbers mean little.
- Forgetting length. A long run at a high bit rate fails even when every wire is fine.
Assumptions and limits
The tool judges the physical layer only. Resistance and voltage cannot show a wrong bit rate, wrong message IDs or a gateway that isolates the bus. The vehicle-specific CAN access point is not known to the tool.
All limits are typical values and are editable. Sources: application report SLLA270, a published J1939 introduction, and the SAE J1939-11 and J1939-14 titles on sae.org for the bit rates (checked 2026-11-03). The voltage levels, terminator, cable lengths and connector pins could not be read in an official text, because ISO 11898-2 and SAE J1939-13 are sold standards. Verify them against the current official text for your bus.