The P1871 diagnostic trouble code is designed to let you know that the computer isn’t sure what gear the transmission should be in. It factors in the RPM, throttle position sensor/TPS position and vehicle speed sensor /VSS data. The engine is running above 300 RPM with a quarter throttle, and the vehicle speed is below 7 mph, a P1871 DTC will be saved.
How Serious is the Code?
The P1871 OBDII diagnostic trouble code is fairly serious, because the transmission will be locked into 2nd gear (limp mode), torque converter clutch operation will be prohibited and maximum line pressure will be commanded. These conditions are designed to protect the transmission parts from further damage. However it can cause damage to other components that would have been otherwise unaffected.
Symptoms of a P1871 include:
- Delayed gear changes
- Increased engine RPMs between shifts
- No upshifts beyond 2nd gear/limp mode
- Dirty transmission fluid
- Damaged wiring/connectors
- Failed vehicle speed sensor/VSS sensor
- Failed input shaft speed sensor/ISS sensor
- Failed output shaft speed sensor/OSS sensor
- Failed ECU/TCM
- Valve body issues
How to Diagnose the Code?
In order to properly diagnose a P1871 GM diagnostic trouble code, the condition of the transmission fluid should first be addressed. If it is clean and bright, the wiring and connectors should be inspected for signs of damage. If no problems are found there, various tests will need to be performed to verify the operational condition of the sensors related to this transmission DTC.
What Repairs Will Fix P1871?
- Change the transmission fluid/filter
- Repair damage wiring/connectors
- Replace vehicle speed sensor
- Replace input shaft speed sensor
- Replace output shaft speed sensor
- Repair or replace valve body
- Repair or replace ECU/TCM