What does it mean?
OBD II trouble code P0894 is a generic code that is defined as “Transmission component slipping”, and is set when the PCM (Powertrain Control Module) detects that the rotational speed of one or more rotating components of the transmission is lower than expected.
While the setting parameters of this code vary greatly between applications, all applications use input data from the throttle position sensor, engine speed sensor, vehicle speed sensor, and others to monitor and control the functioning of the automatic transmission. Based on the input data from various engine and transmission sensors, the PCM expects to see various parts of the transmission rotate at speeds that agree with pre-programmed values.
In practice, this means that if the engine is rotating at a given speed, the torque converter should rotate at the same speed. More importantly though, and depending on the currently selected gear, the transmission output shaft should rotate at a speed that corresponds to a pre-programmed value that takes into account the gear ratio reduction that applies to the selected gear, regardless of the engine speed while that gear is selected.
The above is saying a lot, but all it means is that if the engine is rotating at say, 3 000 RPM, all parts of the transmission should rotate at a speed that corresponds to that engine speed, given the currently selected gear. Thus, if the output shaft speed is lower than expected, the PCM will deduce that a transmission component is slipping relative to one or more other components, and it will set code P0894 as a result.
The image below shows the maze of fluid passages in a relatively uncomplicated automatic transmission valve body. Note that even a partial blockage of any of the passages can cause a valve shuttle or ball check valve not to move freely, which is often a cause of code P0894.