CAN bus (Controller Area Network) is the nervous system of a modern car, connecting electronic control units (ECUs) for the engine, transmission, security system and other components. Problems with resistance on the CAN bus - one of the most common causes of failures in on-board electronics, which manifest themselves in the form of errors on the dashboard, unstable operation of systems, or complete failure of individual modules. For example, if your car suddenly stops recognizing the key, refuses to start, or displays the error “No communication with the ECU,” there is an 80% chance that the CAN bus will be the culprit.
Many car owners and even some repairmen confuse problems with the CAN bus with malfunctions of individual sensors or units, wasting time and money on replacing working parts. Meanwhile, checking CAN bus resistance with a multimeter takes no more than 10 minutes and allows you to accurately determine where the problem lies: a broken wiring, short circuit, faulty terminator or damaged connector. In this article we will figure out what resistance values are considered normal, how to take measurements correctly, and what to do if the parameters are outside the acceptable limits.
The CAN bus operates on the principle of differential signal transmission: data is transmitted over two wires - CAN_High (CAN+) and CAN_Low (CAN-), which minimizes interference. For the network to work correctly, both wires must be connected terminators - resistance resistors 120 Ohm, installed at the ends of the bus (usually in the engine ECU and another end module, for example in the comfort unit). If the resistance deviates from the norm, it leads to signal reflections, data transmission errors and vehicle malfunctions.
CAN bus resistance is the first parameter that is checked when diagnosing problems with on-board electronics. But there are nuances here: for example, some cars (especially premium brands) may use terminators with a different rating, and in networks with a “star” or “ring” topology, the measurement principles are different. Next we will analyze all these cases in detail.
Normal CAN bus resistance values
In most cars, resistance between wires CAN_High and CAN_Low should be 60 ohm (for a network with two 120 Ohm terminators connected in parallel). However, this rule does not always work. Here are the key points to consider:
1. Standard bus topology: If the CAN network is built according to the classical scheme (linear bus with terminators at the ends), the resistance between CAN_High and CAN_Low will be equal 55–65 Ohm. This is due to the fact that two 120 Ohm resistors connected in parallel give exactly this value. A deviation of more than 5 ohms in any direction is already considered a malfunction.
2. Star or ring topology: On some models (eg BMW, Mercedes-Benz with the system FlexRay) other connection schemes may be used. In this case, the resistance may differ. For example, in networks with a star topology, terminators are installed in each node, and the total resistance will be higher - up to 80–100 Ohm.
3. No terminators: If one of the terminators is damaged or disconnected, the resistance between CAN_High and CAN_Low will increase to 120 Ohm (if one resistor does not work) or tends to infinity (if both do not work). This is one of the most common causes of CAN bus failures.
4. Short circuit: If the wires CAN_High and CAN_Low shorted to each other or to ground, the resistance will be close to 0 ohm. This situation leads to a complete network failure and requires immediate repair.
It is important to understand that regulations may vary depending on the vehicle model. For example, on some Toyota and Honda terminators with resistance are used 110 Ohm, and on Volkswagen and Audi - standard 120 Ohm. To avoid mistakes, always check the wiring diagram for your specific vehicle.
Reasons for resistance deviations from the norm
If, when measuring the CAN bus resistance, you receive a value outside the permissible limits, the reasons may be as follows:
- 🔧 Broken wiring: Damage to one or both wires (
CAN_HighorCAN_Low) leads to an increase in resistance or its complete absence (infinity). Most often, breaks occur in places where the harnesses are kinked (for example, near the doors or under the hood). - ⚡ Short circuit: Shorting the wires to each other or to the vehicle ground causes the resistance to drop to zero. This can happen due to abrasion of the insulation, moisture or mechanical damage.
- 🔌 Faulty terminators: Resistors (terminators) can burn out due to voltage surges, corrosion or manufacturing defects. If one terminator does not work, the resistance will increase to 120 Ohms, if both, the bus will completely fail.
- 🔄 Bad contacts in connectors: Oxidation, corrosion, or loose connections in CAN bus connectors (such as the OBD-II diagnostic connector) result in unstable resistance. Sometimes the problem only appears when there is vibration or heat.
- 🚗 Connecting non-standard equipment: Installing alarms, multimedia systems or other devices connected to the CAN bus may compromise its integrity. For example, an incorrectly installed alarm system can “break” the bus by adding an extra terminator.
One of the most insidious reasons is hidden wiring damage. For example, wires may be pinched during vehicle assembly at the factory or damaged during repairs. In this case, the resistance may periodically “float”, which complicates diagnosis. It is also worth paying attention to transition resistances in connectors - they can add several Ohms to the total bus resistance, which in total gives a deviation from the norm.
Another nuance: on some cars (for example, Ford or Mazda) The CAN bus can be divided into several segments with different data rates (for example, High-Speed CAN and Low-Speed CAN). In this case, the resistance of each segment is checked separately.
⚠️ Attention: If the CAN bus resistance is equal to 0 ohm, do not turn on the ignition under any circumstances! This can lead to failure of the control units due to a short circuit. First, find and eliminate the cause of the short circuit.
How to correctly measure CAN bus resistance with a multimeter
To check the resistance of the CAN bus, you will need a digital multimeter with a resistance measurement function (ohmmeter). Here are the step-by-step instructions:
1. Turn off the power: Turn off the ignition and remove the battery terminal (negative). This is necessary to avoid false readings due to active control units.
2. Locate the CAN bus wires: Most cars have wires CAN_High (usually orange or yellow) and CAN_Low (usually green or white) can be found:
- In the diagnostic connector OBD-II (pins 6 and 14).
- In the connectors of control units (for example, engine ECU).
- In the wiring harnesses under the dashboard.
3. Connect the multimeter: Set the multimeter to resistance measurement mode (200 ohm range) and connect the probes to the wires CAN_High and CAN_Low.
4. Take readings:
- Norm: 55–65 Ohm (for standard topology).
- One terminator does not work: ~120 Ohm.
- A break or two non-working terminators: infinity (OL).
- Short circuit: 0 Ohm or close to zero.
5. Check the resistance of each wire to ground: This will help identify the short circuit. Connect one multimeter probe to CAN_High or CAN_Low, and the second - to the mass of the car. There must be resistance infinite (OL). If there is any value, it indicates a closure.
6. Check the integrity of the wires: Set the multimeter to "test" mode and check each wire for breaks. To do this, connect the probes to the ends of one wire (for example, CAN_High in the OBD connector and in the ECU). If the circuit is closed, you will hear a beep.
Turned off the power (removed the terminal from the battery)|
I found the CAN_High and CAN_Low wires in the OBD connector|
Measured the resistance between CAN_High and CAN_Low|
I checked the resistance of each wire to ground|
I checked the wires for a break
-->
If you take measurements in the diagnostic connector OBD-II, please note that some adapters (for example, for ELM327) may affect the readings. For accurate diagnosis, it is better to connect directly to the wiring harnesses.
On vehicles with multiple CAN segments (for example, High-Speed CAN and Low-Speed CAN) The resistance of each segment is checked separately. For example, on Volkswagen Golf MK6 High-Speed CAN (engine, transmission) and Low-Speed CAN (comfort, climate) have different terminators and require separate testing.
Diagnosis of faults by resistance
Depending on the multimeter readings, you can determine the type of fault and localize the problem. Below is a table with the most common cases:
| Resistance between CAN_High and CAN_Low | Resistance CAN_High → ground | Resistance CAN_Low → ground | Probable Cause | Actions |
|---|---|---|---|---|
| 55–65 Ohm | OL (infinity) | OL (infinity) | CAN bus is OK | The problem is not the tire. Check control units or firmware. |
| 120 Ohm | OL | OL | One terminator does not work | Locate and replace the faulty terminator (usually in the ECU or comfort unit). |
| OL (infinity) | OL | OL | Break in one or both wires / both terminators do not work | Test the wires for continuity. Check the terminators in the control units. |
| 0 ohm | 0 ohm or low | 0 ohm or low | Short circuit between CAN_High and CAN_Low or to ground | Find the short circuit, restore the insulation or replace the harness. |
| 80–100 Ohm | OL | OL | Non-classical topology (star, ring) or additional terminators | Check the vehicle's electrical diagram. Make sure there are no unnecessary terminators. |
If the CAN bus resistance is normal, but problems with the on-board electronics persist, the reasons may be as follows:
- Damage to wire shielding (interference from other systems).
- Malfunction of one of the control units (for example, the computer “sticks” on the bus).
- Problems with power supply or mass of control units.
For example, if the engine ECU does not receive stable power (+12V or mass), it can block the CAN bus, which manifests itself in the form of communication errors. In this case, the bus resistance will be normal, but it will not work.
⚠️ Attention: On some vehicles (eg BMW E60) when the battery is disconnected, the adaptation settings of the control units are reset. After restoring the CAN bus, you may need to retraining the throttle, power windows or other systems.
Finding and eliminating breaks and short circuits
If the test shows an open circuit or short circuit, it is necessary to localize the problem area. Here's how to do it:
1. Broken wire:
- Use the multimeter in dial mode.
- Connect to one end of the wire (for example, CAN_High in the OBD connector) and ring it along its entire length, connecting sequentially to the connectors of the control units.
- The place where the signal disappears is the break point.
2. Short circuit:
- Disconnect all control units from the CAN bus (disconnect the connectors).
- Check the resistance between CAN_High/CAN_Low and mass at each site.
- Areas where the resistance is not equal OL, contain a closure.
3. Damaged insulation:
- Carefully inspect the wiring harnesses for chafing, melting or traces of moisture.
- Pay special attention to areas where wires pass near sharp body edges or moving parts (for example, near pedals).
4. Faulty terminators:
- Terminators are usually located in the outermost control units of the CAN bus (e.g. engine control unit and comfort unit).
- To check the terminator, disconnect the unit connector and measure the resistance between the contacts CAN_High and CAN_Low on the block itself. It should be equal 120 Ohm (or another denomination if a non-standard terminator is used).
If you find a break, restore the circuit by soldering or replacing a section of wire. If there is a short circuit, replace the damaged harness or restore the insulation using heat shrink tubing. If the problem is in the terminator, it can be replaced with a similar resistor with the same value (usually 120 Ohm, 0.25–0.5 W).
If you can't find the break, try applying power to the wire CAN_High voltage 5V through a resistor 1 kOhm and test it with a voltmeter. The voltage drop will indicate the location of the break.
Common errors when diagnosing a CAN bus
Even experienced technicians sometimes make mistakes when checking the CAN bus. Here are the most common of them:
- 🔌 Resistance measurement with ignition on: Active control units may distort multimeter readings. Always turn off the power before checking.
- 📊 Ignoring network topology: Not all cars have a classic linear tire. On some models (for example, Audi A6 C7) a star topology is used and the resistance will be different.
- 🔧 Continuity of wires without disconnecting units: If control units are not disconnected, their internal circuits may create false current paths, masking breaks.
- 🔍 Failure to check shielding: Damaged CAN bus shielding may cause interference even if the resistance is normal. Always inspect harnesses for damage.
- 🚗 Ignoring non-standard equipment: Alarms, multimedia systems and other devices connected to the CAN bus can add unnecessary terminators or disrupt the integrity of the network.
Another common mistake is replacing terminators without checking. For example, if the CAN bus resistance is 120 Ohms, many people immediately change the terminators without checking whether there is a break in the wires. As a result, the problem remains, and money and time are wasted.
It is also worth remembering that on some cars (for example, Mercedes-Benz with the system CAN-C) are used active terminators, which cannot be replaced with a regular resistor. In this case, if the terminator malfunctions, the entire control unit must be replaced.
What happens if you leave the CAN bus with the wrong resistance?
If the CAN bus resistance deviates from the norm, this leads to the following consequences:
- Communication errors between control units (for example, error "No Communication with ECU").
- Unstable operation of systems: The engine may stall, the transmission may shift jerkily, and the climate control may not work correctly.
- Data loss: Some units may freeze or reset (such as power windows or memory seats).
- Failure of control units: In the event of a short circuit, a breakdown of the ECU input circuits is possible, which will require expensive repairs or replacement.
Practical examples of CAN bus repair
Let's consider several real cases of CAN bus malfunctions and their elimination:
1. Volkswagen Passat B6: CAN bus resistance - 120 Ohm
- Reason: The terminator in the comfort unit (J519) did not work.
- Repair: Replacing the comfort unit or soldering a new resistor (120 Ohm) to the board.
- Bottom line: Resistance returned to 60 ohms, communication errors disappeared.
2. Toyota Camry 2015: CAN bus resistance - 0 Ohm
- Reason: Short circuit of wires CAN_High and CAN_Low in the harness under the driver's seat (the insulation is frayed).
- Repair: Replacement of the damaged section of the harness, restoration of insulation.
- Bottom line: The resistance returned to normal, the car started without errors.
3. BMW E60: CAN bus resistance - infinity
- Reason: Wire break CAN_Low in the connector under the steering column.
- Repair: Resoldering the contact in the connector, checking the integrity of the wire.
- Bottom line: Communication between blocks has been restored, errors have disappeared CAN Timeout.
4. Ford Focus 3: CAN bus resistance - 80 Ohm
- Reason: An abnormal alarm has added an extra terminator.
- Repair: Removing an extra resistor from the alarm circuit.
- Bottom line: Resistance returned to 60 Ohms, climate control operation restored.
In all these cases, the key to successful repair was accurate diagnosis. Without checking the CAN bus resistance, technicians could spend hours replacing control units or reflashing without solving the underlying problem.
Before replacing any control units be sure to check the CAN bus resistance. In 80% of cases, the problem lies not in the ECU, but in the wiring or terminators.
FAQ: Frequently asked questions about CAN bus resistance
Is it possible to drive with a faulty CAN bus?
Technically it is possible, but it is highly undesirable. A faulty CAN bus leads to malfunctions in the vehicle's systems: sensors may fail, the gearbox may not operate properly, the ABS or stabilization system may turn off. In some cases, the car may simply not start. In addition, long-term driving with a damaged CAN bus can lead to failure of the control units.
How to check the CAN bus if you don’t have a multimeter?
Without a multimeter, it is impossible to carry out an accurate diagnosis, but you can indirectly assess the condition of the tire:
- Connect a diagnostic scanner (for example, ELM327). If it does not see the control units or produces communication errors, the problem may be in the CAN bus.
- Check whether the error lights on the instrument panel are illuminated (for example,
Check Engine,ABS,ESP). The simultaneous lighting of several indicators often indicates problems with the CAN bus. - Inspect the wiring harnesses for visible damage (fraying, melting).
However, for accurate diagnosis, a multimeter or oscilloscope is necessary.
What to do if the CAN bus resistance is normal, but errors remain?
If the resistance between CAN_High and CAN_Low within 55–65 Ohms, but problems with the on-board electronics persist, check the following:
- Power supply and weight of control units: Make sure that all ECUs are receiving stable voltage
+12Vand reliable mass. - Shielding integrity: Damaged shielding may cause interference even if the resistance is normal.
- Block firmware: Sometimes CAN bus malfunctions are caused by incompatible firmware (for example, after chip tuning).
- Contacts in connectors: Oxidation or poor contact in the control unit connectors can interrupt communication.
If all else fails, one of the control units may be faulty (for example, it is stuck on the bus). In this case, only replacing or repairing the unit will help.
Is it possible to solder terminators in control units yourself?
Technically yes, but this requires experience with a soldering iron and knowledge of circuit design. Terminators are usually SMD resistors on the control unit board. If you decide to solder yourself:
- Use a soldering iron with no more power than 30 W with a thin sting.
- Do not overheat the board as this may damage the traces or other components.
- After soldering, check the CAN bus resistance and make sure there are no short circuits.
- On some blocks (for example, Bosch or Siemens) terminators can be integrated into the microcircuit, and their replacement is impossible without specialized equipment.
If you are not confident in your skills, it is better to contact a specialist or replace the control unit assembly.
What tools are needed to diagnose the CAN bus?
To fully diagnose the CAN bus you will need:
- Digital multimeter (with resistance measurement and continuity testing function).
- Diagnostic scanner (for example, Launch X431, Autel or even simple ELM327 to read errors).
- Oscilloscope (optional, for analyzing CAN bus signals in case of complex faults).
- Wiring repair kit (soldering iron, solder, heat shrink tube, electrical tape).
- Car wiring diagram (to know where the terminators are located and how the wires are laid).
For most tasks, a multimeter and a scanner are sufficient, but in difficult cases (for example, when searching for hidden breaks), an oscilloscope may be required.