Corroded, loose, or damaged battery terminals and cable ends causing voltage drops under load; Open or high-resistance circuit in one of the module's power feed wires, often due to chafing, rodent damage, or a failed splice; Blown or intermittent fuse in one of the power feed circuits, sometimes caused by a downstream short; Faulty power distribution module or relay that supplies voltage to one of the inputs; Internal failure within the control module itself, where the voltage sensing circuit misreads one input; Poor ground connection for the module, which can skew voltage readings on both power inputs; Voltage drop across a shared connector or junction point that affects only one of the two feeds

DTC codes / U30xx · Network wiring and bus faults / U3018

U3018Control Module Input Power Circuit A/B Correlation

Q: What does code U3018 mean?

Control Module Input Power Circuit A/B Correlation. U3018 is a generic network communication code indicating that a control module has detected a mismatch between its two primary power input circuits, typically a main power feed and a redundant or backup feed. The module compares voltage levels on these inputs and sets the code when the correlation between them falls outside the expected range, often due to a voltage drop, open circuit, or high resistance on one of the feeds.

U3018 is a generic network communication code indicating that a control module has detected a mismatch between its two primary power input circuits, typically a main power feed and a redundant or backup feed. The module compares voltage levels on these inputs and sets the code when the correlation between them falls outside the expected range, often due to a voltage drop, open circuit, or high resistance on one of the feeds.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaNetwork wiring and bus faults

Symptoms you may notice

Illumination of the malfunction indicator lamp (MIL) or a dedicated warning message on the dashboard
Intermittent or complete failure of one or more electronic systems controlled by the affected module
No noticeable symptoms besides the warning light in many cases, especially if the module continues to operate on the remaining good power feed
Random electrical glitches such as flickering lights, false warnings, or modules dropping offline temporarily
Difficulty starting or stalling if the affected module is critical to engine or transmission operation
Multiple unrelated trouble codes stored in various modules due to voltage instability

Common causes, most likely first

Corroded, loose, or damaged battery terminals and cable ends causing voltage drops under load
Open or high-resistance circuit in one of the module's power feed wires, often due to chafing, rodent damage, or a failed splice
Blown or intermittent fuse in one of the power feed circuits, sometimes caused by a downstream short
Faulty power distribution module or relay that supplies voltage to one of the inputs
Internal failure within the control module itself, where the voltage sensing circuit misreads one input
Poor ground connection for the module, which can skew voltage readings on both power inputs
Voltage drop across a shared connector or junction point that affects only one of the two feeds

How to diagnose and fix it

Perform a thorough visual inspection of battery terminals, cables, and the module's power feed wiring for corrosion, looseness, or damage
Use a digital multimeter to measure battery voltage at the terminals and compare it to the voltage present at each of the module's power input pins under load
Check the fuses and relays that supply each power input circuit, replacing any that are blown or intermittent after identifying the cause
Perform a voltage drop test on the power feed wires and ground circuits to locate excessive resistance, repairing or replacing the affected wire or connector
Inspect and clean the module's ground connections, ensuring they are tight and free of corrosion
If all external circuits test good, substitute a known-good module or perform a bench test to confirm internal failure before replacement
After repairs, clear the code and verify that it does not return during a test drive with electrical loads cycled on and off

What Most Techs Miss on U3018

A voltage drop that only appears under high electrical load, such as when the cooling fan or rear defroster kicks on, can set U3018 intermittently and be missed by static voltage checks.
The code does not always mean the module is faulty; a weak battery or failing alternator can cause both power inputs to fluctuate in a way that triggers the correlation error.
Corrosion inside a fuse box or power distribution center often affects only one of the two feeds, creating the exact voltage mismatch this code describes.
Some vehicles use a shared power feed that splits at a splice; a high-resistance splice can cause one input to read lower than the other without any visible wire damage.
After replacing a module, the new unit may require programming or initialization, and failing to do so can leave U3018 stored even though the hardware is good.

Can I keep driving with U3018?

If the vehicle runs and drives normally with only the warning light on, you can drive it to a repair shop, but avoid heavy electrical loads and long trips until the fault is corrected. If the engine stalls, multiple systems fail, or the vehicle enters limp mode, stop driving and have it towed to prevent unsafe operation.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.