OBD-II trouble code
U0007: High Speed CAN Communication Bus (-) Low
CAN-Low pulled below where it should rest. Unlike a short to battery voltage, a short to ground does not damage anything — which makes this the one bus fault that stays put long enough to be found by halving the network.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $100 – $900
- DIY difficulty
- Shop recommended
What does U0007 mean?
CAN-Low is the conductor that is meant to be pulled down. At rest it sits near 2.5 volts alongside CAN-High, and a transmitting module drives it toward 1.5 volts to create the difference that represents a dominant bit. U0007 sets when the module finds it sitting lower than it should — most often held near chassis potential by a short to ground.
What that does to the network follows from the arithmetic of a differential pair. Hold CAN-Low at ground and the difference between the two conductors is permanently positive by more than the dominant threshold, whatever any module is trying to transmit. Receivers see a dominant state that never ends. Recessive bits, which are how a bus signals idle and how arbitration resolves, simply cease to exist. Nothing can complete a message, because a message needs both states.
The part that separates this code from its short-to-battery counterparts, and the reason it is worth reading rather than skimming, is what it does not do. CAN transceivers are specified to survive continuous shorts across a wide voltage range, and ground is comfortably inside it. The chips are not being cooked. They will keep trying, indefinitely, for as long as the ignition is on. There is no cumulative damage clock running, no argument for having the car towed rather than moved under its own power if it will start, and no reason to fear that a few minutes of diagnosis will turn a wiring repair into a module repair. Practically, that means the fault is stable — and a stable fault is the one thing that makes systematic bus diagnosis actually work.
Use that. Disconnect the battery and measure resistance from CAN-Low to chassis ground; on a healthy vehicle it should be high, and with this fault present it reads near zero. Then divide the network in half at a connector or splice pack, measure again, and keep halving toward whichever side still reads low. On an intermittent fault that method is useless because the reading changes for reasons unrelated to what you unplugged. Here it is reliable, and a bus with a dozen modules can be narrowed to one branch in a handful of measurements. This is the code where binary search genuinely earns its reputation.
One measurement caution specific to this line. Many networks use split termination, where the terminating resistor is divided into two halves with the centre point coupled to ground through a capacitor to improve noise performance. That capacitor is normal and it will make a resistance reading to ground drift while it charges. Let the reading settle before judging it, and take the stable value rather than the first number on the display.
Because chassis is available to the harness at practically every clip and bracket, the causes are mechanical rather than electrical: a fastener through the loom, a pinch under a seat rail, a crushed section at a door boot, or water sitting in a connector with a grounded shell. Look at what has been fitted or disturbed recently before reaching for a wiring diagram.
Treat the vehicle as not driveable while the bus is down, since the systems that depend on it are running on defaults or not at all.
Common causes
- CAN-Low shorted to chassis ground at a clip, bracket, or grommet where the loom is pinched
- Fastener driven through the harness during accessory, trim, or seat installation
- Crushed or abraded conductor at a door, tailgate, or bulkhead pass-through
- Water standing in a connector with a grounded shell, bridging the pin to the body
- Internal short inside a module pulling CAN-Low toward ground
- Corroded splice pack bridging CAN-Low to an adjacent ground circuit
- Damaged terminal contacting the connector body
- Rodent damage exposing conductor against a panel
- Unsealed previous repair resting against chassis metal
Symptoms
- Complete and consistent loss of communication that does not come and go
- Scan tool unable to establish a connection with any module on the affected bus
- Multiple warning lamps illuminated simultaneously and remaining on
- No-start or crank-no-start where the immobiliser cannot be reached
- Instrument cluster dead or frozen
- Transmission locked in a default gear
- Fault that reproduces identically every single time the ignition is switched on
- Symptoms beginning immediately after seat, trim, or accessory work
Diagnostic steps
- 1.Disconnect the battery and measure resistance from CAN-Low to chassis ground. Near zero confirms the short; a high reading sends you elsewhere.
- 2.Let the reading settle before judging it. Networks using split termination couple the centre of the terminator to ground through a capacitor, which makes the first number on the display drift.
- 3.Exploit the stability of this fault. Because a short to ground does not damage transceivers, the fault does not move, and halving the network while watching the resistance is a reliable method here rather than a hopeful one.
- 4.Split the bus at a major connector or splice pack, measure each side, and continue halving toward whichever side still reads low until one branch is isolated.
- 5.Do not use the 60-ohm reading across the pair as a clearance. Both terminators remain connected to each other through a short to ground, so that value can look correct on an unusable bus.
- 6.Ask what has recently been fitted, removed, or disturbed. A screw through a loom or a harness trapped under a seat rail is the most frequent physical cause and the timing usually identifies it.
- 7.Inspect along sills, under seats, and at every clip and bracket the harness passes, since chassis metal is available to it almost everywhere.
- 8.Check connectors for standing water, especially any with a metal shell or grounded backshell.
- 9.Where isolating one module restores the reading, measure at that module's own connector before condemning it, so a fault in the branch is not blamed on the module.
- 10.After repair, confirm both a normal CAN-Low-to-ground resistance and a normal reading across the pair before returning the vehicle.
Repair cost
$100 – $900
Diagnosis is $150 to $300, and this is one of the few bus faults where that figure is predictable rather than open-ended, because the fault holds still and the network can be halved methodically instead of chased. Wiring or connector repair to clear the short is $150 to $600 depending on how buried the contact point is. If a module is internally shorting the line, replacement with programming is $400 to $900 or more, higher on European and luxury platforms. There is no collateral damage bill on this code — transceivers tolerate a short to ground — so what you pay is the repair itself rather than the consequences of having driven with it.
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.