P1871 — Undefined Gear Ratio

Diagnostic algorithm

1. Read all stored DTCs and freeze frame data with a manufacturer-capable scan tool; note related codes and conditions when code set.
2. Conduct a visual inspection: check transmission fluid level/condition and inspect harness/connectors for damage or corrosion; repair as needed.
3. With scan tool, log input (turbine) and output speeds while performing a road test or on a lift (safely) through several gears. Verify the calculated gear ratio vs. expected gear ratios for that transmission.
4. If speeds do not change or ratio is incorrect, test each speed sensor circuit: verify reference voltage, ground, and signal using a multimeter/oscilloscope. Check tone ring/reluctor for damage or missing teeth.
5. Repair or replace any faulty sensors, wiring, connectors, or grounds. Clear codes and retest to see if ratio returns to normal.
6. If electrical tests and sensors are good but ratio still incorrect, perform hydraulic/pressure tests (line pressure at idle and during shifts) and inspect valve body/shift solenoids. Replace or rebuild valve body/solenoids if they fail specs.
7. If pressure and control components are within specifications yet the problem persists, suspect internal mechanical damage (clutch packs, planetary gears, or torque converter). Consider removal and internal inspection or full transmission rebuild/replacement.
8. After repairs, clear codes, perform a full road test with live data logging and verify no reoccurrence. Check for TSBs or software updates for the specific GM vehicle and apply if recommended.