Cause
Faulty transfer case control module
The module that manages range shifts and four-wheel drive engagement — reading the selector, driving the transfer case actuator, checking the result. It is the least likely member of its own circuit codes: connectors, wiring and the actuator all fail far more often than the module judging them.
Suspect it last, and configuration first. A replacement module usually needs coding to the vehicle, and an uncoded one produces faults identical to a dead one.
Typically specialist work. Needs equipment or training most home workshops do not have.
Some of these codes mean you should stop driving
3 of the 6 codes this can cause indicate a fault where continuing to drive risks damage or is unsafe. Which one you have matters more than the cause does — read the code's own page before deciding.
Codes this can cause (6)
Codes where this is a likely explanation first, rather than merely a possible one.
- P279A Transfer Case Gear High Incorrect Ratio Stop driving High
- P279B Transfer Case Gear Low Incorrect Ratio Stop driving High
- P279C Transfer Case Gear Neutral Incorrect Ratio Stop driving High
- U0102 Lost Communication With Transfer Case Control Module Moderate
- U0403 Invalid Data Received From Transfer Case Control Module Moderate
- U0439 Invalid Data Received From All Terrain Control Module Moderate
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
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