Undefined Gear Ratio
On Buick vehicles, P1871 is logged when the ECU detects a fault in the powertrain: “Undefined Gear Ratio”. P1871 is a manufacturer-specific Buick code (the "1" after the letter marks an OEM-defined code), so its meaning applies to Buick and may differ on other makes.
Typical triggers for a powertrain code like P1871 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.
When a Buick 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 P1871 stay cleared after the flash. Genuine faults should always be repaired, not just hidden.
Softechpro Solutions does DPF/EGR/AdBlue/DTC off + checksum for 260+ ECU families — from €98.99/mo.
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