The P0398 code indicates that the engine control module has detected a voltage signal from the Cylinder 1 pressure sensor that is above the normal operating range. This typically signifies an open circuit or a short to power in the sensor's electrical path.
The sensor may have an internal electrical failure causing it to report a high voltage signal.
Frayed, broken, or shorted wires in the sensor circuit can cause the signal to drift high or open.
Moisture or corrosion in the sensor plug can increase resistance or cause a signal interruption.
If the sensor is not seated correctly, it may provide inaccurate readings that the PCM interprets as a circuit fault.
An internal failure in the ECM's processing circuit for the pressure sensor, though this is rare.
No, this is an electrical circuit code. It means the computer sees a voltage that is too high, not necessarily that the physical pressure in the cylinder is low.
No. It is very uncommon for the PCM to cause a 'circuit high' code; the issue is almost always the sensor itself or the wiring connecting it to the computer.
Generally no. P0398 monitors the electrical circuit of the sensor, whereas vacuum leaks typically cause lean codes or idling issues.