What it means
Voltages on input power lines A and B do not correlate as expected — one is significantly different from the other.
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 monitors input power circuits A and B as separate supplies and checks that their voltages agree as expected. U3018 sets when the A-to-B voltage relationship is outside the permitted range, which can signal a weak connection or supply fault on either circuit.
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 record freeze-frame data to see whether the A/B mismatch occurred during starting, charging, or another voltage change.
- Compare live readings for input power circuits A and B under the same operating conditions, if the scan tool provides them.
- Inspect both supply fuses, connectors, and harnesses, paying attention to corrosion, loose terminals, or heat damage on either circuit.
- Measure voltage at the module on circuits A and B, then perform voltage-drop tests under load to locate a high-resistance feed.
- Compare both circuit readings with the manufacturer's specification and check for a separate gateway or CAN fault only if the supplies match.
Common fixes
- Clean and secure a loose or corroded connection on circuit A or B.
- Replace a blown supply fuse after checking why it opened.
- Repair the high-resistance or damaged wire on the circuit that drops voltage.
- Correct a confirmed gateway or CAN fault if circuits A and B test within specification.
Can I keep driving?
Because unequal supplies can make the module unstable or unavailable, avoid driving if powertrain or safety functions are affected. If limp mode, shutdown, or critical warnings occur, stop in a safe place.
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.