P1870

Transmission Component Slipping

Car Fault Codes GMC Transmission

Can you still drive? With care, short trips only

Who can fix it? Needs a specialist or dealer

Typical repair time 1 hour – 10 hours

Typical cost SAR 250 to 5,000

What it means

This is a GMC manufacturer-specific code, and the same number can mean something different on other makes. The computer sets it when it commands torque converter clutch (TCC) lockup and then finds engine speed still higher than transmission input speed by more than allowed. On the 4L60-E and 4L65-E it is commonly linked to worn TCC valve bores in the valve body or a worn converter clutch, and heat and heavy loads speed it up.

Models covered: GMC models with the 4L60-E/4L65-E automatic, mainly 1990s–2000s (e.g. Sierra, Yukon).

Symptoms4

Engine rpm wavering at steady speed or when towing.
A shudder like driving over rough pavement at medium speeds.
Transmission fluid running hot under load.
Check engine light on.

Possible causes5

Worn TCC valve bores in the valve body causing pressure loss.
A worn lockup clutch inside the torque converter, especially with frequent towing.
Low or heat-burned transmission fluid.
A weak TCC PWM solenoid.
Low overall line pressure from pump wear (less common).

How to diagnose5

Check fluid level and condition; dark fluid with a burnt smell shows overheating.
Watch TCC slip speed and fluid temperature on a scan tool during steady driving.
Make sure there is no solenoid electrical code such as P1860.
Have a transmission specialist check line pressure and inspect the valve body.
If slip continues with a good valve body, a worn converter is likely.

How to fix5

Correct the fluid level and change fluid and filter if burnt.
Replace the TCC PWM solenoid if it is confirmed weak.
Repair the valve body with bore repair parts or replace it.
Replace the torque converter, and consider an extra transmission cooler if you tow often.
Clear the codes and monitor slip and temperature on a test drive to confirm the code does not return.

Repair cost

0–500

500–2,000

2,000–5,000

5,000+

In Saudi riyals; the highlighted part is the expected range for this fault.

Fluid service or a solenoid is the low end, while a valve body or torque converter needs specialist transmission work that pushes the cost to the top of the range.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions

Does towing and heavy hauling cause this code?

It speeds it up. Loads raise fluid temperature and stress the lockup clutch, so wear in the valve body and converter shows up sooner.

Does the transmission need a full rebuild?

Not always. Often a valve body repair or a new torque converter is enough; a rebuild is for cases with other shifting faults too.

How do I protect the transmission after the repair?

Keep the fluid level and quality right, change it on schedule or sooner if you tow often, and watch its temperature in summer.

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This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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