Possible causes

– Trailer towing may set code
– Check transmission fluid level
– Internal transmission fault

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P1870 GMC Description

Transmission is in 4th gear with Torque Converter Clutch (TCC) engaged and the Engine Control Moduel (ECM) has detected excessive slippage when comparing the input shaft speed to the output shaft speed

GMC Technical Service Bulletins for P1870 code