Code Definition

P1812 — Transmission 4-Wheel Drive Mode Select Circuit Failure

System: Powertrain  |  Type: Manufacturer-Specific  |  Make: Generic

About P1812

P1812 is a manufacturer-specific powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P1812 is stored, the module has detected an electrical-circuit fault in the wiring, connector, or driver associated with the named component.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

P1812 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

This DTC is set when the Powertrain Control Module (PCM) detects an invalid or inconsistent signal from the 4WD mode select circuit, which is responsible for communicating the selected transfer case mode (e.g., 2HI, 4HI, 4LO) to the PCM. The mode select input typically comes from a rotary switch or selector module and is interpreted via discrete voltage levels or a multiplexed signal. If the signal is missing, out of range, or does not correspond to valid 4WD mode logic, the PCM sets P1812.

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