My '02 Blazer won't shift into 3d gear
 

My '02 blazer has 50k miles on it and now it won't shift into third gear. I replaced the Mass Airflow Sensor but the problem persists. I't will shift from first to second but when it tries to shift into third it misses and it revs. Depending on the weather or if going up hill it will hit the shift but otherwise it won't shift. the error is P1870 but that a generic for transmission slippage. Does anybody have any guidance? :icon_fryingpan: