U0003 Code: High Speed CAN Communication Bus (+) Open

What the U0003 code means

The CAN network uses two twisted wires: CAN high (+) and CAN low (-). At rest both sit near 2.5 V. When a message is sent, the high line rises toward 3.5 V and the low line falls toward 1.5 V. There is a 120 ohm termination resistor at each end of the network, and the two in parallel add up to 60 ohms.

The U0003 code flags an open circuit on the high speed CAN bus (+). With one line open, the network may keep working partway or stop working in one section.

The high speed network usually ties together the engine, transmission, ABS and steering. The medium or low speed network ties together the comfort modules (body, doors, climate control, radio). Knowing which modules are affected helps you aim the diagnosis.

Diagnostic method, in order: 1) battery and alternator (low voltage is the number one cause of bursts of U codes); 2) the module list on the scan tool to see which one does not respond; 3) power, fuses and grounds of the module that does not respond; 4) network resistance at the OBD2 connector (about 60 ohms between pins 6 and 14 with the battery disconnected); 5) module connectors and wiring, especially in damp areas (under the carpets, in the doors, in the trunk).

U0003 code symptoms

• Many warning lights on at the same time.
• Several U codes in different modules.
• Limp mode, or the engine does not start (high speed network).
• Comfort functions that do not respond (low speed network).

Likely causes of the U0003 code

1. Damaged network wiring: cut, rubbed through or wet.
2. A shorted module taking down the network. You find it by unplugging modules one at a time.
3. Termination resistor missing or damaged. The total resistance goes from 60 to 120 ohms.
4. Connectors with water or corrosion, especially under the carpets, in the doors and in the trunk.
5. Accessory connected to the network: alarms, trackers, OBD2 adapters.
6. Weak battery, which causes communication errors.

How to diagnose the U0003 code step by step

1. Measure the resistance between OBD2 pins 6 and 14 with the battery disconnected: about 60 ohms. A reading of 120 means a termination is missing or a section is open. A reading near 0 means the lines are shorted together.
2. Measure the voltage of each line with the key on. CAN high should be 2.5 to 3.5 V and CAN low 1.5 to 2.5 V. A reading of 0 V or 12 V points to a short.
3. Unplug the modules one at a time until the network starts working again.
4. Check the wiring in areas of rubbing and moisture.
5. Remove accessories connected to the network.

How to fix the U0003 code

• Repair the wiring, keeping the wires twisted as they were.
• Replace the module that is taking down the network.
• Remove poorly installed accessories.
• Clear the codes in all the modules.

Technical reference data

• Resistance between pins 6 and 14: about 60 ohms.
• Terminations: 120 ohms each.
• CAN high: 2.5 to 3.5 V. CAN low: 1.5 to 2.5 V.
• Approximate values only: check the manufacturer data for your vehicle.

Brand-specific notes

An open line can leave part of the network alive, which is why the symptoms can look odd. The U0003 code points you toward a cut wire or a loose connector on the CAN high line.

After the repair, clear the codes in every module, because many of them keep old U codes from the time the fault was present. Accessories installed later (alarms, GPS trackers, OBD2 adapters left plugged in) are a frequent cause of network trouble.

Frequently asked questions about U0003