Low, burnt or contaminated transmission fluid causing clutch slip; Faulty or dirty input or output speed sensor, or its wiring and connector; Worn clutch packs or bands causing internal slip; Shift solenoid or pressure control solenoid sticking or failing; Valve body wear or internal hydraulic leakage, which varies by transmission type; Faulty transmission control module or outdated software calibration.

P1871

Undefined Gear Ratio.

Buick
Powertrain
Manufacturer code
High severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
8: AreaTransmission
71: FaultThe specific fault in that area

What it means

P1871 on a Buick means the transmission control module has calculated a gear ratio from the input and output speed signals that does not match any of the valid ratios for the transmission. In effect the gearbox is not in a recognised gear, which suggests slip, a speed signal fault or an internal hydraulic or mechanical problem. The exact trigger conditions vary by model and transmission, so check the manufacturer data for your vehicle.

Symptoms

Gearbox warning light or check engine light on
Harsh, delayed or flared gear changes
Transmission slipping or engine revving without matching acceleration
Gearbox stuck in one gear or in limp-home mode
Erratic or dead speedometer or poor shift quality

Possible causes

Low, burnt or contaminated transmission fluid causing clutch sliplikely
Faulty or dirty input or output speed sensor, or its wiring and connectorlikely
Worn clutch packs or bands causing internal slippossible
Shift solenoid or pressure control solenoid sticking or failingpossible
Valve body wear or internal hydraulic leakage, which varies by transmission typepossible
Faulty transmission control module or outdated software calibrationrare

How to diagnose

Read all stored codes and freeze frame data, noting speed, gear commanded and fluid temperature when the fault set
Check transmission fluid level, condition and smell, following the correct procedure for the model
Inspect the speed sensor wiring and connectors for damage, corrosion, chafing or fluid ingress
Compare live input speed, output speed and vehicle speed data on a road test to see when the ratio goes wrong
Check for other transmission codes, such as solenoid, pressure or speed sensor faults, and deal with those first
Test the speed sensors and solenoids against the manufacturer's resistance and signal specifications
Check for a software update or calibration bulletin for the model and transmission
Carry out line pressure and further internal checks if the electrical side tests good

How to fix

Correct the fluid level and renew the fluid and filter if it is burnt or contaminated, using the specified type
Repair damaged wiring or connectors, or replace a faulty speed sensor
Replace a faulty shift or pressure control solenoid
Repair or replace the valve body if wear or leakage is confirmed
Overhaul or replace the transmission if clutches or bands are worn
Update the control module calibration if an outdated one is a known cause for the vehicle

Can I keep driving?

High severity

Avoid driving beyond what is necessary, as continued slip can quickly cause serious and costly internal damage to the transmission. Have it checked promptly, and do not tow or drive far if it slips or sticks in gear.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.