What this code means

The TCM detected transmission slipping during cruise conditions. Once the vehicle reaches cruise state, the TCM monitors the transmission ratio. If slipping is detected, P1870 sets and line pressure is elevated to stop further slippage. This code is extremely common on GM vehicles with the 4L60E transmission.

Symptoms

• Check Engine Light
• Harsh 1-2 shift
• Transmission slipping
• Fourth gear may become unavailable
• Higher RPMs than normal
• Poor fuel economy
• General poor transmission performance

Common causes

• Worn or failed TCC (Torque Converter Clutch) solenoid valve causing pressure loss (most common on 4L60E)
• Dirty, degraded, or contaminated transmission fluid
• Defective or sticking valve body shuttles
• Excessive wear in TCC isolator/regulator valve bore
• Worn transmission clutch packs
• Low transmission fluid level
• Internal transmission seal leaks

Diagnostic steps

1. Check transmission fluid level and condition
2. Monitor transmission slip rate with scan tool
3. Test TCC solenoid valve
4. Inspect valve body for worn bores
5. Test line pressure at all gear ranges
6. Check for TCC apply and release

Repair notes

On 4L60E transmissions, the TCC solenoid is almost always the root cause. Change fluid and filter, then replace TCC solenoid. If code returns, valve body bore wear may require a remanufactured valve body or transmission rebuild.

Other Powertrain codes on this vehicle