Damaged, chafed or pinched CAN (-) wire shorted to ground; Corroded or water-damaged connector or terminal on the low speed CAN network; Faulty control unit pulling the CAN (-) line low internally; Poor repair or aftermarket accessory wiring (alarm, stereo, towbar module) interfering with the bus; Low battery voltage or poor earth connection causing unstable communication; Faulty gateway or junction connector joining the network branches.

U0025

Low Speed CAN Communication Bus (-) Low.

Generic
Network
Generic code
Medium severity

How this code is built

U: SystemNetwork
0: Defined bySAE, the same on every car
0: AreaNetwork area 0
25: FaultThe specific fault in that area

What it means

U0025 is the generic SAE J2012 code for the low speed CAN communication bus (-) line being low. A control unit has detected that the CAN low line, on a low speed (fault tolerant) network, is held at a lower voltage than it should be, for example shorted to ground. This usually affects body and comfort systems and can stop modules talking to each other.

Symptoms

Warning lights on the dashboard, often several at once
Central locking, windows or mirrors working intermittently or not at all
Instrument cluster gauges or displays behaving oddly or going blank
Interior or exterior lights not responding correctly
Other U-codes for lost communication with various modules
Battery drain if modules fail to go to sleep

Possible causes

Damaged, chafed or pinched CAN (-) wire shorted to groundlikely
Corroded or water-damaged connector or terminal on the low speed CAN networklikely
Faulty control unit pulling the CAN (-) line low internallypossible
Poor repair or aftermarket accessory wiring (alarm, stereo, towbar module) interfering with the buspossible
Low battery voltage or poor earth connection causing unstable communicationpossible
Faulty gateway or junction connector joining the network branchesrare

How to diagnose

Read all stored codes from every module and note which ones set U0025 or other communication codes, then record freeze frame data if available
Check battery condition and charging voltage, and inspect main earth connections, as low voltage can cause false communication faults
Visually inspect wiring and connectors along the low speed CAN network, especially in door shuts, under seats, and near areas with previous repairs or water ingress
Check for any aftermarket accessories fitted and disconnect them one at a time to see if the fault clears
Using the manufacturer wiring diagram for the specific model, identify which modules sit on the low speed CAN and where the network splits
Measure CAN (-) line voltage and resistance to ground with the ignition off and modules disconnected one by one to isolate the shorted branch
Use an oscilloscope to view the CAN signal and confirm the low line is stuck low
Unplug suspect modules in turn and recheck the line to find one that is pulling it low

How to fix

Repair or replace the damaged CAN (-) wiring section using proper soldered or crimped and sealed repairs, keeping the twisted pair arrangement
Clean, repair or replace corroded connectors and terminals, and cure the source of water ingress
Replace or reprogram a confirmed faulty control unit as specified by the manufacturer procedure
Remove or correct faulty aftermarket wiring that interferes with the bus
Restore battery health and repair poor earth connections, then clear codes and confirm they do not return

Can I keep driving?

Medium severity

The car can often still be driven short term, but the loss of communication may disable body, lighting or safety-related functions, so get it checked soon. If warning lights for brakes, airbags or lighting are affected, limit driving until it is resolved.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.