P1871
Undefined Gear Ratio.
- Oldsmobile
- Powertrain
- Manufacturer code
- Medium 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 an Oldsmobile means Undefined Gear Ratio. The transmission control module compared input shaft speed with output shaft speed and got a ratio that does not match any gear the transmission should be in. This usually points to slip, a speed signal fault or an internal transmission problem. Exact thresholds and the speed sensors used depend on the model and transmission.
Symptoms
- Malfunction indicator lamp or transmission warning lamp on
- Transmission slipping or engine flaring between gear changes
- Harsh, delayed or erratic shifts
- Transmission stuck in one gear (limp mode)
- Speedometer or shift behaviour erratic
- Poor acceleration or reduced fuel economy
Possible causes
- Low or degraded transmission fluid, or a fluid level that is wronglikely
- Faulty or dirty input or output speed sensor, or a damaged sensor signallikely
- Damaged wiring or connectors to the speed sensors or transmission harness (chafing, corrosion, water ingress)possible
- Worn clutches or bands causing internal slippossible
- Sticking shift or pressure control solenoid, or valve body wear causing low line pressurepossible
- Torque converter clutch slip or converter faultrare
- Faulty transmission control module or software issuerare
How to diagnose
- Read all stored and pending codes and freeze frame data, noting road speed, gear commanded and fluid temperature when the code set
- Check transmission fluid level, condition and smell as per the model's procedure, and look for metal or clutch debris
- Inspect the transmission harness, connectors and speed sensor wiring for damage, corrosion and leaks
- Compare live input and output speed sensor data and the commanded gear against actual gear ratio on a road test
- Check the speed sensors for debris, damage and correct fitting, and test their resistance or signal against the model's specification
- Check for other transmission or engine codes that could be causing the fault, and clear and road test to see if it returns
- Carry out a line pressure test and solenoid checks if the model procedure allows
- Check for any applicable service bulletins or calibration updates for the transmission control module
How to fix
- Correct the fluid level or replace degraded fluid and filter with the specified type
- Replace faulty input or output speed sensors
- Repair or replace damaged wiring, connectors or terminals
- Replace faulty shift or pressure control solenoids, or repair the valve body
- Overhaul or replace the transmission if clutches, bands or the torque converter are worn
- Update or replace the transmission control module only if testing confirms a fault or a known calibration issue
Can I keep driving?
Medium severity
Short journeys are possible if the transmission shifts normally, but get it checked soon, as continued slip can cause expensive internal damage. If it slips badly, flares or sticks in gear, stop driving and have it inspected.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.