What it means
Voltages on input power lines B and C do not correlate as expected. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.
At a glance
- Usually lights the warning light of the system that lost communication
- The engine or gearbox may go into limp mode
How it works
The module compares input power circuits B and C to confirm that their voltages have the expected relationship. U3019 sets when the B-to-C comparison is outside range, indicating that one supply may have excessive resistance or otherwise be failing.
Common causes
Most likely first.
- One supply line has high resistance
- Gateway module routing CAN messages incorrectly
- CAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Scan all modules and save freeze-frame data to identify whether the B/C mismatch appeared during starting, charging, or a load change.
- Compare live readings for input power circuits B and C under the same conditions, if available.
- Inspect the fuses, terminals, connectors, and wiring for both circuits for corrosion, looseness, heat damage, or a blown fuse.
- Measure voltage at the module on B and C and perform voltage-drop tests under load to identify which feed loses voltage.
- Compare the readings with the manufacturer's specification and check gateway routing or CAN only if both power circuits test correctly.
Common fixes
- Clean and secure a loose or corroded connection on circuit B or C.
- Replace a blown fuse after checking for the cause.
- Repair the high-resistance wire or terminal on the supply circuit that fails its voltage-drop test.
- Correct a confirmed gateway or CAN fault if B and C supplies match the manufacturer's specification.
Can I keep driving?
The B/C supply mismatch may make the module lose communication or cause dependent systems to enter limp mode. Avoid driving if a safety or powertrain function is affected, and stop if the vehicle shuts down or behaves abnormally.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.