What does it mean?
OBD II fault code P0727 is a generic code that is defined as, “Engine Speed Input Circuit No Signal”, and is set when the PCM (Powertrain Control Module) detects the absence of an input signal from the engine speed sensor. NOTE: On some vehicles, the engine input sensor is also known as the “transmission input speed sensor”, and might be located on the transmission casing where it can interact with a reluctor wheel inside the transmission.
On some other applications, such as many, if not all VW and related VAG-group vehicles, the crankshaft position sensor does double duty as the engine speed sensor when the engine is running. The control system of a modern automatic transmission requires an enormous amount of input data from many different sources to manage gearshifts effectively, and an actual gearshift is the result of complex calculations that are based on a large variety and range of operating parameters.
In simple terms though, a transmission control system needs to know four things to function, these being the engine speed, the engine load, the currently selected gear, and the position of the throttle plate i.e. if the throttle is opening or closing. These four parameters represent the absolute minimum in terms of input data any transmission control system requires to work.
However, of these four parameters, the engine speed is arguably the most important (all other things being equal), since all shift points are based on the actual engine speed. For instance (and regardless of the selected operating mode), transmission control systems are programmed to initiate gearshifts at pre-defined engine speeds to a) prevent component damage, b) minimize exhaust emissions, and c) to maximize engine performance while using the least amount of fuel, at the same time.
Thus, accurate, valid, and plausible input data on the actual engine speed is a crucially important parameter to ensure efficient transmission control. Therefore, should a failure, defect, or malfunction in the input speed sensor or its associated electrical circuits occur that deprives the PCM or TCM (Transmission Control Module) of an input signal, these modules will recognize that they cannot control the transmission effectively.
Note that while such a failure or malfunction will usually cause code P0727 to set on the first failure, whether (or not) a warning light will also illuminate depends on the vehicle make and model. On some vehicles, a warning light will illuminate on the first failure, while on others, two or more failures need to occur on two consecutive trips before a warning light will illuminate.