CAN Bus Diagnosis (High-Speed, ISO 11898-2)

Reference values: 500 kbit/s typical · 2 × 120 Ω → 60 Ω · Recessive H/L 2.5/2.5 V · Dominant H/L 3.5/1.5 V · OBD pin 6 = CAN-H, 14 = CAN-L, 4 = chassis GND, 5 = signal GND, 16 = B+

1. Prepare

  • Battery: ~12.6 V at rest, ~13.8–14.6 V charging. Smart charging may differ. Use a maintainer.
  • Check fuses, power and grounds of affected modules. A module without supply looks like a bus fault.
  • Back-probe only. Never pierce CAN wires.
  • Tools: full-system scan tool, multimeter, 2-channel scope, OBD breakout, wiring diagram.

2. Scan

Save a full-system scan before clearing codes.

Code Meaning
U0001 High-speed CAN communication bus
U0100 Lost comm. with ECM/PCM
U0101 Lost comm. with TCM
U0121 Lost comm. with ABS
U0140 Lost comm. with BCM
U0155 Lost comm. with instrument cluster
  • Many modules lose one module → check that module's power, ground and connector.
  • Many modules lose many → bus fault, go to steps 4–7.
  • Historic codes only → intermittent, wiggle-test while monitoring.

3. Map and inspect

  • From the wiring diagram, note: buses and gateway, location of both terminators, splice points, and whether the OBD port is on this bus or only on the gateway.
  • Inspect for corrosion, pushed-back terminals, chafing, water ingress and aftermarket devices. Unplug aftermarket devices first.

4. Termination resistance (pin 6 ↔ 14)

Bus must be dead: wait for all modules to sleep, or disconnect the battery.

Reading Meaning
0–2 Ω H shorted to L
2–50 Ω Too low. ~40 Ω = third terminator
50–55 Ω Slightly low, check for moisture/leakage
55–65 Ω Normal
65–110 Ω High-resistance connection
110–135 Ω One terminator missing or main bus open toward one end
Open Both terminators missing, open near test point, or port not on bus
  • 60 Ω only proves the main bus. A broken stub to one module still reads 60 Ω, so measure at that module.
  • If ~120 Ω: unplug an end module and measure its harness side. ~120 Ω = that module's terminator is faulty; open = wire to it is broken.
  • Some makes split termination differently. Check the diagram.

5. Shorts (bus dead)

Measure Fault if
Pin 6 → 4 ~0 Ω: CAN-H to ground
Pin 14 → 4 ~0 Ω: CAN-L to ground
Pin 6 / 14 → 16 ~0 Ω: short to B+ (confirm with voltage in step 6)

Both lines should read similarly high (kΩ–MΩ) to ground.

6. Voltages (ignition on, multimeter)

Condition CAN-H CAN-L
Normal, active ~2.6 V (2.5–3.0) ~2.4 V (2.0–2.5)
Normal, idle 2.5 V 2.5 V
H–L short 2.5 V 2.5 V
H to ground 0 V pulled low
L to ground pulled low 0 V
Either to B+ ~battery V ~battery V

Low-speed fault-tolerant CAN (ISO 11898-3, ≤125 kbit/s) is different: no 60 Ω across the pair; recessive H ≈ 0 V, L ≈ 5 V; dominant H ≈ 3.6 V, L ≈ 1.4 V; can run on one wire.

7. Oscilloscope

Ch A = CAN-H, Ch B = CAN-L, 1 V/div, 10–20 µs/div (bit = 2 µs at 500 kbit/s). Healthy: mirrored square edges, ~2 V differential when dominant.

Symptom Cause
Ringing/overshoot Missing terminator, long untwisted section
Rounded edges Moisture, failing transceiver
One line flat That line shorted
Both lines shifted, curved edges One line open to a module
Lines move together Poor ground, common-mode noise
Bus constantly busy Babbling module, unplug one at a time

CAN FD: same levels, faster data phase (≥2 Mbit/s), use a faster timebase.

8. Isolate, repair, verify

  1. Monitor at OBD port; unplug modules one at a time (aftermarket first) until readings normalise.
  2. If no change, split the harness at splice points and halve until the section is found.
  3. Repair with correct wire, sealed staggered joints, twist kept to the joint. Replace corroded terminals and fix the leak.
  4. Replacement modules: match part number (incl. internal terminator) and code as required.
  5. Verify: ~60 Ω, clean waveform, full scan with no current U-codes, road test, confirm the car goes to sleep.

Typical values only. Always confirm with the manufacturer's service information.