Communication codes — the U-codes — are some of the most intimidating faults on a modern vehicle, and some of the most misdiagnosed. "Lost communication with the ABS module" rarely means a dead ABS module. Modern cars network dozens of control units over a CAN bus, and when one drops off — or the whole bus falls silent — the codes cascade across every module that was talking to it. The skill is reading that pattern: figuring out whether you've lost one module or a segment of the bus, then testing power, wiring, and termination in the right order. This guide is the general method behind every U-code.
What U-codes actually mean
- U0xxx "Lost Communication With …" — a module that expected to hear from another module on the bus stopped receiving its messages. The code names the silent module, not necessarily the broken one.
- U0001 / bus-related — high-speed CAN communication bus faults.
- "Bus off" — a module detected so many errors it took itself off the bus to protect it.
- U-codes with an FTB suffix — the sub-code narrows the failure mode (see what a failure type byte is).
Read the pattern: one module lost (likely that module's power, ground, or branch wiring) versus many modules lost at once (a bus-segment, gateway, power, or ground problem shared by all of them).
The diagnostic sequence
- Scan every module, not just the engine. Write down which modules report faults and which are completely unreachable. The list of who-can't-hear-whom points to the fault location.
- Check battery voltage and grounds first. Low system voltage and bad grounds set communication codes across multiple modules at once — the single most common cause. See why low voltage causes multiple DTCs.
- Verify power and ground at the silent module. A module with no power looks "lost" on the bus but is electrically fine. Confirm its feed and ground before touching the bus.
- Measure termination resistance. Two 120Ω terminators in parallel should read about 60Ω across CAN-H and CAN-L at the OBD port (key off). Outside ~54–66Ω confirms a physical bus fault — see the 60-ohm rule.
- Inspect the bus wiring — CAN is a twisted pair; look for opens, shorts (to each other, to power, or to ground), water intrusion, and corroded connectors at the affected branch.
- Isolate a module dragging the bus down. If the bus is shorted or a module has failed internally, disconnecting modules one at a time can restore communication and identify the culprit.
- Consider the gateway. Many vehicles route CAN through a central gateway; a gateway fault can split the network.
Common causes
- Low voltage / bad grounds — the leading cause of multi-module U-codes, especially after a weak battery or jump start.
- Water and corrosion in connectors or a module — a classic source of intermittent bus dropouts.
- Damaged CAN wiring — chafed twisted pair, a pinched harness, or rodent damage.
- A failed module shorting or flooding the bus, or losing its own power/ground.
- For a single ABS no-response specifically, see fixing U0121 ABS no-response.
FAQ
Does a "lost communication" code mean the module is bad?
Usually not. The code names the module that went silent, but the cause is most often that module's power/ground, the bus wiring, or low system voltage — not a failed module. Confirm those before replacing anything.
What should the CAN bus resistance read?
About 60Ω across CAN-H and CAN-L at the OBD port with the key off, because the two 120Ω terminators sit in parallel. A reading near 120Ω means one terminator/branch is missing; near 0Ω means a short; well above 66Ω means an open.
Why did I get communication codes after a dead battery or jump start?
Low voltage during cranking or a jump can cause modules to miss messages and store U-codes across the network. Restore a healthy battery and charging system, clear the codes, and recheck — many will not return.
How do I find which module is causing a bus-wide fault?
After confirming power, ground, and termination, disconnect modules one at a time while watching the bus. If communication is restored when a particular module is unplugged, that module (or its branch wiring) is dragging the bus down.