U0025 indicates a fault on the low-speed Controller Area Network (CAN) communication bus. This code is set when the Powertrain Control Module (PCM) or other gateway module detects that messages are not being properly transmitted or received on the low-speed CAN bus, which typically handles body control functions such as climate control, door locks, and instrument cluster.
Physical damage to the CAN low or CAN high wires, chafing against engine components, or corrosion can cause communication failures. This is the most frequent cause of U0025.
Each end of the CAN bus requires a 120-ohm termination resistor. If one or both resistors are open or out of specification, signal reflection can disrupt communication.
A failed module connected to the low-speed CAN bus can drag the network down, preventing proper communication between all devices.
The CAN transceiver chip inside a control module can fail, causing that module to no longer properly transmit or receive CAN messages.
Insufficient voltage or poor ground connections to modules on the CAN bus can cause intermittent or complete communication loss.
The PCM itself can rarely fail and cause low-speed bus communication issues, but this should be diagnosed only after all other more common causes are ruled out.
It depends on which systems are affected. If only non-critical comfort functions are impacted, the vehicle may be drivable, but you may experience issues with climate control, locks, or instrument cluster. If communication loss affects safety-critical systems, the vehicle may not start or could have reduced functionality.
Rarely. While some technicians may suggest PCM replacement as a first attempt, the PCM is almost never the cause of low-speed CAN bus communication faults. The PCM itself rarely fails in a way that takes down the entire low-speed bus. Spending time diagnosing the actual wiring, termination resistors, or individual modules is almost always necessary.
U0025 specifically refers to a fault on the 'low' side (CAN-L) of the low-speed CAN bus, while U0026 indicates a fault on the 'high' side (CAN-H). Both typically stem from similar causes, but pinpointing which wire is affected helps narrow the diagnosis.
Repair costs vary dramatically depending on the root cause. A simple wiring repair or termination resistor replacement may cost a few hundred dollars, while diagnosing and repairing multiple module failures or extensive wiring harness damage can cost significantly more. Accurate diagnosis is essential before estimating repair costs.