Low transmission fluid level or fluid that is degraded or burnt; Worn clutch packs, bands or torque converter clutch slipping internally; Low line pressure from a worn pump, blocked filter or leaking seals; Faulty shift or pressure control solenoid, or a sticking valve body component; Faulty input or output speed sensor giving incorrect readings, or damaged wiring and connectors; Outdated transmission control module calibration where a manufacturer update is known to address the fault.

P1870

Transmission Component Slipping.

GMC
Powertrain
Manufacturer code
High severity

How this code is built

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

What it means

P1870 on a GMC means the transmission control module has detected a transmission component slipping. It sets when the measured relationship between engine speed and transmission input or output speed does not match the gear or clutch commanded, which points to a clutch, band or the hydraulic circuit failing to hold. The exact component monitored depends on the model and transmission fitted.

Symptoms

Engine revs rise during a gear change or under load without matching acceleration
Delayed, soft or flared shifts
Harsh or erratic engagement when selecting Drive or Reverse
Transmission warning light or check engine light on
Transmission stuck in one gear (limp mode)
Burnt smell from the transmission fluid

Possible causes

Low transmission fluid level or fluid that is degraded or burntlikely
Worn clutch packs, bands or torque converter clutch slipping internallylikely
Low line pressure from a worn pump, blocked filter or leaking sealspossible
Faulty shift or pressure control solenoid, or a sticking valve body componentpossible
Faulty input or output speed sensor giving incorrect readings, or damaged wiring and connectorspossible
Outdated transmission control module calibration where a manufacturer update is known to address the faultrare

How to diagnose

Read all stored codes and freeze frame data, noting gear, speed, load and fluid temperature when the fault set
Check the transmission fluid level, condition and smell using the correct procedure for that model, and look for metal debris or burnt fluid
Inspect the transmission wiring, connectors and speed sensor plugs for damage, corrosion or fluid contamination
Look for external fluid leaks around the pan, seals and cooler lines
Use a scan tool to compare engine speed with input and output speed live, and watch for slip in each gear and converter lock-up
Check commanded versus actual line pressure and solenoid operation, following the procedure for that transmission
Test the speed sensor signals and circuit resistance against the manufacturer specification
Check whether a calibration update applies to that vehicle and transmission

How to fix

Correct the fluid level and replace degraded fluid and filter with the specified type
Repair any wiring, connector or leak fault found
Replace a faulty speed sensor or shift solenoid
Repair or replace the valve body, pump or seals if pressure is low
Overhaul or replace the transmission if clutches, bands or the torque converter are worn
Apply a manufacturer calibration update only where one is confirmed as relevant

Can I keep driving?

High severity

Limit driving, as continued slipping can quickly overheat and destroy the transmission. Have it checked as soon as possible and avoid heavy loads or towing in the meantime.

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