GMC P1399

What P1399 means on a GMC

On GMC vehicles, P1399 is logged when the ECU detects a fault in the powertrain: “Wheel Speed Sensor 2 G-Sensor Circuit Intermittent”. P1399 is a manufacturer-specific GMC code (the "1" after the letter marks an OEM-defined code), so its meaning applies to GMC and may differ on other makes.

Common causes & symptoms

Typical triggers for a powertrain code like P1399 include faulty or ageing sensors, wiring/connector faults, a clogged or removed component, or a value drifting outside the ECU's expected window. Symptoms range from a check-engine light only, to limp mode, rough running, higher consumption or failed emissions tests.

P1399 and ECU tuning

When a GMC has had hardware changed for off-road/motorsport use (for example a removed DPF or EGR), the related monitor must be disabled in the ECU map or the code re-appears. Softechpro Solutions performs DPF/EGR/AdBlue/DTC off and recalculates the checksum, so codes like P1399 stay cleared after the flash. Genuine faults should always be repaired, not just hidden.

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