OBD-II trouble code
U0008: High Speed CAN Communication Bus (-) High
CAN-Low has been pulled up toward CAN-High, collapsing the difference between them. The network does not jam — it goes quiet, and modules fall off one at a time as their error counters run out, which is why this fault so often arrives as a rolling, intermittent complaint.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $80 – $1,100
- DIY difficulty
- Shop recommended
What does U0008 mean?
On a high-speed CAN network the message is the gap between two wires, not the voltage on either. Both rest near 2.5 volts; a transmitting module widens the gap by pushing CAN-High up and pulling CAN-Low down, and receivers read the difference. U0008 is stored when a module sees CAN-Low sitting higher than it should.
What that does to the network is the opposite of what a raised CAN-High does, and the distinction is worth holding onto because it predicts the symptoms. Pull CAN-Low up toward CAN-High and the gap between them shrinks toward nothing. The receiver reading high-minus-low sees no difference, which is what an idle bus looks like. So the network is not jammed with a signal nobody can override — it simply appears to have nothing on it. Modules keep transmitting into that silence, and because nothing is being acknowledged, each transmitter counts its own failures. CAN error handling is designed to eject a node that cannot get a message through: after enough consecutive failures a module takes itself off the bus, and depending on the design it may then attempt to recover and try again.
That mechanism is why this code so often presents as an intermittent, rolling fault rather than a clean failure. Modules go quiet at different times because they hit their error limits at different rates, and some of them come back. The owner describes warning lights that appear in waves, gauges that work and then do not, and a car that sometimes starts. It is tempting to chase that pattern as a loose connection. The underlying condition may be perfectly steady, with the intermittency coming entirely from the error counters rather than from the wire.
One short in particular belongs to this code and to no other in the family: CAN-Low touching CAN-High directly. Bridging the two conductors pulls CAN-Low up and CAN-High down until they meet in the middle, which is exactly what this code describes. It also gives you a test that works nowhere else. With the battery disconnected, resistance across the two bus wires should be about 60 ohms, from the two 120 ohm terminating resistors in parallel. A direct short between the wires puts a near-zero path across that measurement, so the meter reads far below 60 ohms rather than at or above it. A low reading on this code is close to a diagnosis on its own.
The diagnostic connector deserves a specific look here, more than it does on the CAN-High codes. In the standard sixteen-pin connector the bus wires are split between the two rows: CAN-High occupies a pin in the upper row, while CAN-Low sits in the lower row two positions away from permanent battery positive. That proximity is not theoretical. Bent probes, a spread terminal, corrosion inside the connector, and above all the growing population of devices left permanently plugged into the port — insurance trackers, logging dongles, dashcam power taps — all put battery voltage in physical reach of the CAN-Low pin specifically. If the vehicle has anything hanging off the diagnostic connector, unplug it before doing anything else.
Common causes
- CAN-Low shorted directly to CAN-High, which pulls the two conductors together
- CAN-Low shorted to battery voltage or a switched power feed
- Damaged, corroded, or spread terminals inside the diagnostic connector, where CAN-Low sits close to permanent battery positive
- An aftermarket dongle, tracker, or logger left plugged into the diagnostic port
- Crushed harness where the twisted pair has been pinched between panels or brackets
- Water in a connector bridging CAN-Low to an adjacent powered pin
- Repair splice made onto the wrong conductor
- A failed transceiver in one module holding CAN-Low up
Symptoms
- Warning lamps that appear and clear in waves rather than all at once
- Modules dropping off the network progressively over seconds or minutes
- Intermittent no-start, with the vehicle occasionally starting normally
- Gauges that work briefly after a key cycle and then freeze
- Scan tool that connects, lists modules, and then loses them mid-session
- A long list of lost-communication codes stored across many modules
- Symptoms that change after the vehicle is switched off and restarted
Diagnostic steps
- 1.Unplug anything connected to the diagnostic port before testing. A tracker, logger, or hardwired accessory at that connector is a common route for voltage to reach the CAN-Low pin specifically.
- 2.With the battery disconnected, measure resistance across the two bus wires. About 60 ohms is normal; a reading far below that indicates the two conductors are shorted together, which is the fault this code describes most often.
- 3.Measure CAN-Low to ground with the key on and compare it against CAN-High. Two wires sitting at the same elevated voltage indicates they are bridged; CAN-Low alone sitting high indicates a short to a power source.
- 4.Inspect the diagnostic connector itself with a light. Look for spread terminals, corrosion, and evidence of probing damage in the row that carries CAN-Low.
- 5.Treat the intermittency as a symptom of error counting rather than of a loose wire. Modules leaving and rejoining the bus at different times is expected behaviour on a collapsed differential and does not imply a connection that comes and goes.
- 6.Record which modules drop and in what order across several key cycles. A consistent sequence points toward a location on the bus rather than toward a single failed module.
- 7.Check for crush damage where the harness passes between panels, through grommets, and behind trim that has been removed for other work.
- 8.Disconnect modules one at a time while watching the two bus voltages. A transceiver holding CAN-Low up will release it when that module is unplugged.
- 9.After repair, confirm every module rejoins the bus and stays on it through several full key cycles, since a node that has gone bus-off may need more than a code clear to return.
Repair cost
$80 – $1,100
The cheapest genuine outcome on this code is free: removing an aftermarket device from the diagnostic port. Diagnosis is $150 to $350, though it is often shorter than on other bus faults because the resistance test across the two wires gives a strong early answer. Repairing a short between the conductors or to a power feed runs $150 to $600 depending on where the damage sits and how much trim has to come out. Module replacement with programming at $400 to $1,000 or more applies only where a transceiver has genuinely failed, which is less common here than on a short to battery voltage — a collapsed differential stresses the network logically rather than electrically.
Estimate your repair
Run the numbers for your vehicle
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.