What does 'CAN-Low shorted to bus (+)' actually mean?

It means the two data wires of the medium-speed bus — CAN-Low and CAN-High — are touching each other. The bus works by reading the voltage difference between those wires, so when they are joined the difference disappears and there is nothing left to read. Repair means finding where they contact and separating them.

Is it safe to drive with U0018?

Usually the car still starts and drives, because the medium-speed bus handles comfort and convenience systems rather than the engine or brakes. You may lose climate control, audio, or accessory functions and some displays may not update. It is best to have it diagnosed soon, since the harness damage that caused it can worsen.

Is U0018 a wiring problem or a bad module?

Most often it is physical wiring damage — a crushed or chafed harness, a moisture-bridged connector, or a backed-out terminal touching its neighbor. A module can short the lines internally, but technicians confirm the wiring first, because replacing a module that wasn't the cause is an expensive mistake.

Why did several features fail together?

When CAN-Low and CAN-High are shorted together, every module on that bus loses communication at the same time. Because those modules share one network, all the features they control can stop together even though each component is fine — the shared bus is the single point of failure.

OBD-II trouble code

U0018: Medium Speed CAN Communication Bus (-) shorted to Bus (+)

The negative (CAN-Low) wire of the medium-speed bus is shorted to the positive (CAN-High) wire. With the two lines tied together the voltage difference collapses and modules on that bus can no longer communicate.

Quick facts

System: Network
Category: Network Communication
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $100 – $800
DIY difficulty: Shop recommended

What does U0018 mean?

U0018 means the negative line — CAN-Low or CAN(-) — of the medium-speed CAN bus is shorted directly to the positive line, CAN-High or CAN(+). A CAN bus carries data as the voltage difference between those two wires: CAN-High swings up while CAN-Low swings down, and modules read the gap between them. When the two wires touch each other, that gap collapses toward a single shared voltage and there is no longer a readable signal, so communication on that bus stops.

Many vehicles use a slower medium-speed bus, separate from the high-speed powertrain network, to link body, comfort, infotainment, and convenience modules where split-second timing matters less. U0018 is the medium-speed counterpart to U0009 on the high-speed bus: it names the specific failure of CAN-Low being shorted to CAN-High on that secondary network. This usually comes from physical wiring damage — a crushed or chafed harness where the two conductors are pressed together, a connector flooded with moisture that bridges adjacent pins, or a backed-out terminal contacting its neighbor — rather than from a module fault.

Because the medium-speed bus generally carries comfort features rather than core driving systems, U0018 tends to be less severe than the equivalent high-speed fault. The car usually still starts and drives, but you may lose climate control, audio, displays, or power accessories, and several lost-communication U-codes can set together. It is traced electrically — inspecting the harness and measuring bus resistance and voltage to find where the two lines are joined — rather than by replacing parts on a guess.

Common causes

CAN-Low and CAN-High wires crushed or chafed together in the harness
Moisture or corrosion in a connector bridging the CAN(-) and CAN(+) pins
Backed-out terminal contacting the adjacent bus terminal
Pinched harness from prior repair, accident, or a fastener clamping the bundle
Damaged splice where both bus wires are joined
Rodent or road-debris damage mashing the two conductors together
A failed module internally tying the two bus lines together

Symptoms

Comfort/convenience features stop working (climate, infotainment, accessories)
Driver displays freeze, blank, or show fault messages
Multiple lost-communication U-codes stored with U0018
Scan tool cannot reach modules on the medium-speed bus
Intermittent feature loss that tracks with vibration or moisture
Engine usually still starts and runs normally

Diagnostic steps

1.Record all stored codes in every module to see which medium-speed modules dropped off and help localize the short.
2.Identify which modules sit on the medium-speed bus for the specific vehicle using wiring diagrams.
3.With key off and battery disconnected, measure resistance between CAN-High and CAN-Low; a very low reading between them confirms the two lines are shorted together.
4.Measure bus voltages key-on: if CAN-High and CAN-Low read nearly the same voltage instead of separating, the lines are tied together.
5.Inspect the harness for crush, chafe, and pinch points and check connectors for moisture and backed-out terminals that could bridge the two wires.
6.Repair the short to separate the conductors, then clear the codes and confirm the affected modules reappear on the scan tool and U0018 does not return.

Repair cost

$100 – $800

Diagnosis to locate where the two lines are joined commonly runs $100-$250. A wiring or connector repair to separate CAN-High from CAN-Low is often $150-$550. If a failed module is shorting the bus internally, replacement with programming can run $400-$800+, higher on luxury and European platforms.

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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.

Related codes

U0010 U0011 U0016 U0017 U0009 U0140