---
title: 'CAN Bus Diagnosis (High-Speed, ISO 11898-2)'
url: 'https://jeppo.eu/en/auto/diagnosing-a-can-h-can-l-network'
markdown: 'https://jeppo.eu/en/auto/diagnosing-a-can-h-can-l-network.md'
lang: en
date: '2026-10-15'
description: "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 Monitor at OBD port; unplug modules one at a time (aftermarket first) until readings normalise. If no change, split the harness at splice points and halve until the section is found. Repair with correct wire, sealed staggered joints, twist kept to the joint. Replace corroded terminals and fix the leak. Replacement modules: match part number (incl. internal terminator) and code as required. 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."
---

# 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.*

---

## Navigation

- Next: [Official portals for technical information (EU)](https://jeppo.eu/en/auto/official-portals-for-technical-information-eu.md)