OBD-II trouble code
U0016: Medium Speed CAN Communication Bus (-) Low
CAN-Low pinned down looks to the network exactly like a message that never stops. Every node treats it as a permanently busy bus, error counters climb, and the network shuts itself off — which is why this fault produces a completely dead bus rather than a flaky one.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $900
- DIY difficulty
- Shop recommended
What does U0016 mean?
CAN-Low is the conductor that gets driven *downward*. At rest it idles near 2.5 volts alongside CAN-High; to signal a dominant bit, a transmitting module pulls it down toward roughly 1.5 volts while pushing CAN-High up. The receiving modules read only the difference between the two.
That detail is what makes U0016 behave differently from the other faults in this family. If CAN-Low is held near ground by a short, the network does not simply see a distorted signal — it sees a differential voltage that never returns to the idle state. In CAN terms that is a dominant bit that never ends. Every controller on the network interprets it as another node transmitting continuously, so no controller may start a message of its own, and every controller's internal error counter begins to climb. Once those counters pass their limit the transceivers take themselves off the network entirely. The protocol calls this bus-off, and it is a designed safety behaviour rather than a failure: a network that cannot arbitrate reliably is required to stop rather than deliver corrupted data.
The practical signature is therefore total, not partial. Where a marginal network gives intermittent features and a scan tool that reaches modules some of the time, this fault gives nothing at all — no communication with anything on that network, no intermittency, no improvement when the vehicle warms up or dries out. That absoluteness is diagnostically useful, because it rules out the whole category of marginal-connection faults straight away. A fault that comes and goes is not this one.
It also shapes the fastest route to the answer. Because a single shorted node holds the entire network down whether or not it is trying to transmit, disconnecting modules one at a time and watching for the network to come back to life is unusually decisive here. The moment the offender is unplugged, everything else starts talking again.
Common causes
- CAN-Low conductor shorted to ground through chafe, crush or a pierced harness
- Module with an internally shorted transceiver holding the network dominant
- Water intrusion into a connector bridging the CAN-Low pin to a grounded shell or pin
- Corroded splice pack or junction connector on the body network
- Harness damage from accident repair or aftermarket installation work
- Terminal displaced from its cavity and contacting a grounded surface
- Damaged module connector where a retaining clip has broken and allowed movement
Symptoms
- Complete loss of communication with every module on the medium-speed network
- Scan tool reports no response from that network at all, rather than partial responses
- Multiple body and comfort systems dead simultaneously and consistently
- No intermittency — the fault is present every time, warm or cold, wet or dry
- Warning lamps illuminated for systems that cannot report their status
- Displays blank or frozen with no update
- Engine starts and runs normally
Diagnostic steps
- 1.Confirm the fault is constant rather than intermittent. Total, repeatable loss of the network fits this code; a fault that comes and goes points elsewhere in the family and should be investigated as a marginal-connection problem instead.
- 2.Measure CAN-Low to ground with the ignition on. A reading held near zero rather than idling around 2.5 volts confirms the dominant-state condition.
- 3.Disconnect the battery and check CAN-Low to ground for continuity. Direct continuity with everything unpowered means the harness is the fault and no module can be responsible.
- 4.If continuity is normal with the power off, reconnect and disconnect modules one at a time while watching the network. Because a shorted node holds the network down continuously, the recovery when you unplug the right one is immediate and unmistakable.
- 5.Check the network's terminating resistance across the pair with the battery disconnected. The expected reading is roughly 60 ohms from the two 120-ohm terminators in parallel; a very low reading supports a short across or from the pair.
- 6.Where the harness is at fault, split the network at a junction or gateway connector to halve the search area rather than inspecting its full length.
- 7.Inspect junction and splice packs before individual runs. On body networks these shared connection points collect moisture and are a disproportionately common source of hard shorts.
Repair cost
$100 – $900
Isolating the fault is usually quick relative to the rest of this family, because the condition is constant and the disconnect-one-at-a-time method converges fast — expect $100 to $250 in diagnosis. Harness or splice repair is $150 to $500 depending on access. A module whose transceiver has shorted internally costs $400 to $900 with programming, which is where the top of this range comes from.
Estimate your repair
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Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.