Medium Speed CAN Communication Bus
What it means
A module has detected a fault on the medium-speed CAN bus, a secondary network some makers use for comfort and body modules such as the climate control module, the instrument cluster, the radio and the body control module (BCM). It is usually two twisted wires like the high-speed bus, and whether it exists and which modules are on it varies by make. The code is usually stored in another module, and several U codes together point to a problem with this bus or its power supply.
Symptoms5
- Comfort systems such as the A/C, radio or screen stop working.
- Warning messages or lights on the cluster, or some indicators not working.
- Central locking, windows or interior lights not working normally on some vehicles.
- Scan tool cannot communicate with comfort and body modules.
- Lost-communication codes with modules such as the BCM, cluster or climate control.
Possible causes6
- Weak battery or unstable voltage.
- Corroded or wet connector at a module on the bus, especially modules near the doors or under the carpet.
- Open or shorted bus wires from chafing or from installing accessories such as a stereo or screen.
- Poor ground at one of the modules.
- A module with an internal fault, such as the radio or climate control module, disturbing the bus.
- A fault in the gateway module linking this bus to the others, on vehicles that have one.
How to diagnose6
- Read codes from all modules and identify the modules on the medium-speed bus for your vehicle from the manufacturer diagram.
- Check the battery and terminals and measure voltage.
- Check for recent accessories such as a stereo, screen or alarm and disconnect them temporarily.
- Measure resistance between the two bus wires with the battery disconnected and compare with the manufacturer specification, because termination on this bus varies by make.
- Disconnect modules one at a time and watch for communication to return.
- Inspect connectors exposed to water near the doors, the windshield and under the carpet.
How to fix6
- Charge or replace the battery and clean its terminals.
- Clean corroded or wet connectors and fix the water source.
- Repair open or shorted wires, or remove the accessory causing the problem.
- Repair weak ground points.
- Replace the module proven to be disrupting the bus, then program and code it.
- Clear codes in all modules and confirm the code does not return.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
The cost is low for a battery, a wet connector or an added accessory, rises with wiring tracing, and is highest if a module must be replaced and programmed.
Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026
Frequently asked questions
Does every vehicle have a medium-speed CAN bus?
No. Some makers use it for comfort and body modules, while others name their networks differently, so check the manufacturer diagram to see which modules are on it.
Does this code affect the engine?
Usually not directly, because this bus serves comfort and body systems, but warning messages may appear and some indicators may stop working.
Why does the code appear after installing a stereo or screen?
Tapping an accessory into the bus wiring, or replacing the radio with an incompatible unit, can disturb the bus. Removing the accessory is the first check.
This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.
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