What does it mean?
It used to be that four-wheel drive was relegated to off-road vehicles, which meant that only military vehicles and sport utility vehicles could be found with a transfer case. The transfer case, of course, is the gear box that distributes transmission output between the front and rear axles. Interestingly, there are a lot of cars and trucks that are equipped with transfer cases, not for off-roading, but for performance and stability.
After all, four wheels driving is better than two wheels driving. If your car says “4×4,” “4WD,” or “AWD,” it has a transfer case. One of the major differences that has come about over the last couple decades is the switch from manually-operated transfer cases to electronically-operated transfer cases.
While most 4×4 and 4WD vehicles required you to manually select a four-wheel drive mode, be it 4H or 4L (high and low ranges), via a second shift lever, many have switched over to fully automatic or push-button drive mode selection. Of course, the switch to electronic controls, whether simply a center differential lock or fully-electronic range selection, means the addition of an electronic control unit, the transfer case control module (TCCM).
The TCCM may or may not be part of the ECM or TCM (engine control module or transmission control module), depending on make and model. If the TCCM detects a malfunction, such as an out-of-range sensor or slow actuator, it may set DTC P0800, Transfer Case Control System Malfunction, in ECM memory, which prompts the ECM to illuminate the MIL or CEL (malfunction indicator lamp or check-engine light).
Additional transfer case specific DTCs (diagnostic trouble codes) may also be stored in the ECM, TCM, or the TCCM itself.