What does it mean?
For the engine and transmission to work together, the ECM and TCM (engine and transmission control modules) use a number of sensors to monitor the engine and transmission, such as shaft speed, line pressure, or throttle position. To determine shift patterns, the TCM monitors the shift position switch, sometimes called the PRNDL switch on automatic transmissions, and park/neutral position (PNP) switch. They may even be integrated in the same unit.
The signal from the shift position switch and PNP switch depend on the position of the shift lever. The PNP signal is used to enable or disable engine starting and cruise control. For example, when the transmission is in park or neutral, the PNP switch sends an “ON” signal to the ECM, which enables engine starting and disables cruise control.
If the transmission is shifted out of park or neutral, the PNP switch sends an “OFF” signal to the ECM, which disables engine starting and enables cruise control. On some manual-transmission vehicles, the PNP switch may also be directly connected to the starter circuit, functioning in the same manner to enable or disable starting. The ECM continually monitors the PNP switch, comparing its signal to expected values.
For example, driving at 30 mph, the ECM expects to see the PNP switch in the “OFF” position. Similarly, the ECM expects to see the PNP switch “ON” at the same time as the PRNDL switch is in P or N. If the ECM detects PNP signals with low voltage, it sets a diagnostic trouble code, DTC P0851 – Park Neutral Position (PNP) Switch Input Circuit Low, in system memory.