When a vehicle comes in with multiple U-codes, modules dropping offline, or a scan tool that cannot communicate with half the network, the termination resistance test should be one of the first things you do. It takes thirty seconds, requires only a multimeter, and tells you immediately whether the physical CAN bus has a major integrity fault — a short between the wires, an open circuit, or a missing termination resistor. If the reading is wrong, everything else you find from a scan tool is unreliable because the network itself is compromised. If the reading is correct, you can move on to voltage and signal checks with confidence that the bus hardware is intact.
Why termination resistors exist and what the 60-ohm rule means
A CAN bus is a two-wire differential network — CAN High and CAN Low — that carries data at speeds up to 500 kbps on high-speed powertrain networks. At those speeds, signal reflections at the ends of the bus become a serious problem. Without termination, a voltage pulse travelling down the bus reaches the end of the wire, has nowhere to go, and reflects back — creating interference that corrupts data frames. Termination resistors absorb the signal energy at each end of the bus and prevent reflections.
A standard high-speed CAN network places one 120-ohm resistor at each physical end of the bus — typically inside two specific modules rather than as separate components. The two resistors sit in parallel across the same two wires, and two 120-ohm resistors in parallel produce a combined resistance of 60 ohms. Measuring resistance between CAN High and CAN Low at any point on the bus with the network powered down should produce a reading close to 60 ohms on a healthy network. This is the 60-ohm rule — and any reading significantly different from 60 ohms indicates a physical fault on the bus.
How to measure CAN termination resistance correctly
- Switch the ignition off and wait for the network to go to sleep. Module transceivers that are still active can conduct current across the bus and skew the resistance reading. On most vehicles the network enters sleep mode within a few minutes of ignition off, but some modules stay awake longer. Waiting 30–60 minutes after ignition off ensures all transceivers are inactive. If you need a faster result, disconnecting the battery negative terminal before measuring achieves the same effect — though this resets learned adaptations on some vehicles.
- Access CAN High and CAN Low at the OBD-II diagnostic connector. The OBD-II port is the most convenient measurement point because it is always accessible and is connected to the main CAN bus on most vehicles. CAN High is on pin 6 and CAN Low is on pin 14 of the standard 16-pin OBD-II connector. Set your DMM to the ohms scale — use the 200-ohm range for precision on this measurement. Place one probe on pin 6 and the other on pin 14.
- Read and record the resistance. A healthy high-speed CAN network reads approximately 54–66 ohms at this measurement point — close to the theoretical 60 ohms. Slight variation from exactly 60 ohms is normal due to resistor tolerances, network length, and the measurement point’s position on the bus. Record the reading and compare it against the interpretation table below.
- Wiggle the harness near the OBD-II connector while watching the reading. A resistance that changes significantly during harness movement points to an intermittent fault — a partially broken wire, a loose connector, or a corroded splice near the measurement point. This quick dynamic check can reveal intermittent bus faults that are invisible on a static reading.
Interpreting the reading
| Reading | What it means | Next step |
|---|---|---|
| 54–66 ohms | Both termination resistors present, no short between CAN H and CAN L, bus hardware intact | Bus is physically healthy — move to power/ground checks and CAN voltage/signal testing. See how to test CAN High and CAN Low signals |
| ~120 ohms | One termination resistor missing or one branch of the bus is open — only one 120-ohm resistor is contributing to the measurement | Locate both termination modules using the wiring diagram and test each branch independently to find the open or missing terminator |
| Above 120 ohms or open circuit (OL) | Both terminators are missing, the bus is completely open, or the measurement point is isolated from the network | Check the physical bus wiring for an open circuit, verify the OBD-II connector is wired correctly, and confirm the network is not segmented by a gateway module that is powered down |
| Below 40 ohms | Short circuit between CAN H and CAN L, an additional termination resistor incorrectly wired into the network, or a module transceiver stuck in a dominant state dragging the bus low | Isolate the short by unplugging branches one at a time — resistance rises when the shorted branch is disconnected. Inspect harness routing in that branch for chafing or crush damage |
| Near 0 ohms | Hard short between CAN H and CAN L — wires touching, connector bridged, or harness severely damaged | Physical inspection of the full bus harness routing — look for crush points, chafing against chassis, or connector damage. Isolate branch by branch until resistance rises |
Where the termination resistors are located
On most modern vehicles the termination resistors are built into specific modules rather than installed as separate components. The wiring diagram for the vehicle identifies which modules contain the terminators — typically they are placed at the physical ends of the bus run, often the PCM and ABS module on the powertrain CAN, or the gateway module and the instrument cluster on the body CAN. The exact location matters when isolating a fault because disconnecting a termination module changes the expected resistance reading from approximately 60 ohms to approximately 120 ohms — which is normal once you account for the disconnected resistor.
Some vehicles use dedicated termination connectors — short connectors in the harness that contain a 120-ohm resistor. These can be unplugged individually for testing. Others use termination resistors built into splice packs or junction blocks. Always consult the wiring diagram before disconnecting anything to understand which modules or components carry the termination resistance for the specific network being tested.
How to isolate a fault after an abnormal reading
An abnormal termination resistance reading confirms a physical fault on the bus but does not identify where it is. The isolation procedure works by disconnecting sections of the network one at a time and retesting after each disconnection — the fault location is the disconnection that causes the resistance reading to move toward normal.
For a low reading indicating a short: identify the bus routing from the wiring diagram and locate the major branch points and connectors. Disconnect one branch at a time at a known junction point and retest resistance after each disconnection. When the reading rises — moving away from the short reading toward 60 ohms — the branch you just disconnected contains the short. Inspect the harness in that branch for chafing, crush damage, water intrusion, or a connector that has been forced together incorrectly.
For a high reading indicating an open or missing terminator: locate both termination modules from the wiring diagram. Test continuity on CAN High and CAN Low independently from the measurement point to each termination module. The wire segment that shows an open on continuity testing is where the bus is broken. Check connectors, splices, and harness routing along that segment.
After any repair, remeasure termination resistance before reconnecting anything — confirm the reading has returned to 54–66 ohms. Then reconnect all modules, clear all U-codes, and perform a full vehicle scan to confirm communication has been restored across all modules.
Common mistakes
- Measuring resistance with the ignition on or modules still awake. Active module transceivers can conduct across the bus and produce a lower reading than the true termination resistance. Always measure with the network fully asleep or the battery disconnected. A reading of 30–40 ohms on a vehicle with modules still active may be a healthy bus with awake transceivers, not a fault — wait for sleep before drawing conclusions.
- Unplugging modules randomly to find a fault. Disconnecting modules changes the resistance reading by removing their internal termination — a module that contains a 120-ohm terminator will change the reading from 60 ohms to 120 ohms when unplugged, which can be misinterpreted as finding the fault. Work from the wiring diagram and disconnect at known branch points rather than at module connectors, so you know exactly what each disconnection adds or removes from the measurement.
- Concluding the network is healthy based on resistance alone. A correct resistance reading confirms both termination resistors are present and there is no short or open between CAN H and CAN L. It does not confirm that CAN bias voltages are correct, that the signals are clean, or that all modules are communicating correctly. A network with correct termination resistance can still have signal integrity problems from a module with a failing transceiver. A normal resistance reading means move to the next test — it does not mean the network is fault-free.
- Not accounting for gateway-segmented networks. Many modern vehicles have multiple CAN networks connected through a central gateway module. Measuring resistance at the OBD-II port tests the network segment the port is connected to — typically the diagnostic or body CAN. The powertrain CAN and other segments are only accessible through the gateway when it is powered and functioning. If the gateway is offline, the OBD-II port may show correct resistance on its segment while the powertrain network has a fault that is not visible from the port. Consult the network topology in the service data to understand which segments are accessible from which measurement points.
Frequently asked
My reading is 58 ohms. Is that normal?
Yes. Termination resistors have a manufacturing tolerance — typically plus or minus 5% — and the combined resistance of two resistors with tolerance variation can produce readings anywhere from approximately 54 to 66 ohms on a healthy network. A reading of 58 ohms with all modules asleep and no battery connected is a healthy network. The exact value is less important than whether it falls within the normal range.
Can a failing module cause the termination resistance to read incorrectly?
Yes, in two ways. First, if a module contains a termination resistor and its internal circuitry has failed in a way that disconnects the terminator from the bus, the reading rises to approximately 120 ohms. Second, if a module’s transceiver has failed in a stuck-dominant state — continuously pulling one bus wire low — the reading can drop significantly below 60 ohms even with no wiring fault. The second scenario is identified by the reading returning to normal when the suspect module is unplugged. Be aware that unplugging the module also removes its termination resistor, so interpret the post-disconnection reading accordingly.
Does the 60-ohm rule apply to all CAN networks on the vehicle?
The 60-ohm rule applies specifically to high-speed CAN networks using 120-ohm termination — which covers most powertrain and chassis CAN buses on modern vehicles. Medium-speed CAN and low-speed CAN networks may use different termination values, single-point termination, or no termination resistors at all. LIN bus uses a completely different architecture with no termination resistors. Always check the service data for the specific network being tested before applying the 60-ohm expectation — the wiring diagram will state the termination resistance for each network segment.
I replaced a damaged harness section and the reading is now 60 ohms, but U-codes are still returning. What next?
A correct termination resistance confirms the physical bus is intact — but does not guarantee clean signal quality. After restoring correct termination, check CAN High and CAN Low bias voltages at the OBD-II port with the ignition on — CAN High should sit around 2.5–3.5V and CAN Low around 1.5–2.5V in a healthy idle state. Then scope the differential signal during a module communication event to confirm the waveform is clean and symmetric. Persistent U-codes after a correct termination reading usually point to a module power supply or ground fault, a damaged transceiver in one module, or a second harness fault on a different branch. See how to test CAN High and CAN Low signals.