Low or unstable battery voltage affecting the ACC module's internal electronics.; Corroded or loose electrical connections at the ACC module harness.; Internal hardware failure within the ACC module, such as a faulty circuit board or component.; Software corruption or a need for a module software update.; Water intrusion or environmental damage to the ACC module.; Faulty ground connection for the ACC module.; CAN bus communication errors that interfere with the module's self-test.

DTC codes / C04xx · Brakes, steering and suspension / C0420

C0420Adaptive Cruise Control Module Internal Performance

Q: What does code C0420 mean?

Adaptive Cruise Control Module Internal Performance. C0420 indicates that the adaptive cruise control (ACC) module has detected an internal performance fault, meaning the module's self-diagnostics have found a problem within its own circuitry or processing. The module sets this code when its internal checks fail, often due to voltage irregularities, software corruption, or hardware degradation.

C0420 indicates that the adaptive cruise control (ACC) module has detected an internal performance fault, meaning the module's self-diagnostics have found a problem within its own circuitry or processing. The module sets this code when its internal checks fail, often due to voltage irregularities, software corruption, or hardware degradation.

SystemChassis

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaBrakes, steering and suspension

Symptoms you may notice

The adaptive cruise control system becomes disabled and a warning light illuminates on the dashboard.
The cruise control may not engage at all, or it may disengage unexpectedly while driving.
A message such as 'ACC Not Available' or 'Cruise Control Fault' appears in the driver information center.
The vehicle may revert to conventional cruise control if equipped, or lose cruise functionality entirely.
Often no symptoms besides the warning light, especially if the fault is intermittent.

Common causes, most likely first

Low or unstable battery voltage affecting the ACC module's internal electronics.
Corroded or loose electrical connections at the ACC module harness.
Internal hardware failure within the ACC module, such as a faulty circuit board or component.
Software corruption or a need for a module software update.
Water intrusion or environmental damage to the ACC module.
Faulty ground connection for the ACC module.
CAN bus communication errors that interfere with the module's self-test.

How to diagnose and fix it

Verify battery voltage and charging system performance; recharge or replace the battery if voltage is low.
Inspect the ACC module harness and connectors for corrosion, looseness, or damage; clean and secure as needed.
Check and clean the ACC module ground connection to ensure a solid ground.
Use a scan tool to clear the code and perform a road test to see if it returns; if it does, proceed with further diagnostics.
Check for technical service bulletins (TSBs) or software updates for the ACC module and apply if available.
If all external factors are ruled out, replace the ACC module and perform any necessary calibration or programming.

When C0420 points to more than the module

A weak battery or poor ground can cause the ACC module to set C0420 even though the module itself is healthy, so always verify power and ground integrity first.
Intermittent CAN bus faults from other modules can trigger internal performance codes in the ACC module, so scan all systems for related communication codes.
Some vehicles require ACC module calibration after replacement, which often needs a scan tool with special functions; skipping this step can leave the system inoperative.
C0420 is sometimes stored alongside codes for the cruise control switch or brake pedal position sensor, which can mislead diagnosis if those are not addressed first.
Environmental factors like water intrusion into the module connector can cause internal shorts that set this code, so inspect for signs of moisture or corrosion.

Can I keep driving with C0420?

The vehicle remains safe to drive because the adaptive cruise control system is disabled, but you should avoid using cruise control until the fault is repaired. Have the system diagnosed promptly to prevent further electrical issues.

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.