P1939 Subaru Stx Diagnostic Trouble Code

P1939 Subaru Stx code definition:
P1939 is a kind of OBD-II Diagnostic P-Powertrain Diagnostic Trouble Code for Subaru Stx.

P1939 Subaru Stx is described as Fuel Rail Pressure Sensor Circuit Malfunction. This code is strictly about the incoming signal from the FRP (fuel rail pressure) sensor falling below the calibrated limit for a certain period of time. Other serious codes may cause this code to pop up, and they may come with dangerous drivability issues that may expose you to safety risks and cause damage to the engine in the long run.

Symptoms

Symptoms of a P1939 Subaru Stx diagnostic trouble code may include:

Faulty PCM
The light may stay on or light intermittently as the DPF regenerates. The engine will be sluggish on acceleration.
Poor acceleration
Diminished fuel efficiency

Possible Causes :

Faulty IMRC actuator/sensor
A large vacuum leak would cause a massive amount of un-metered air to enter the intake manifold resulting in an overly lean mixture.
A large air leak at or near the number one oxygen sensor would also cause a lean mixture.

Common Misdiagnoses

Control Module Throttle Actuator Position Performance Problem
Air Conditioning (A/C) Clutch Feedback Circuit High Voltage Problem
Cruise Vehicle Speed/Set Speed Difference Too High Problem
Kick-Down Switch Failed Short Problem
Park/Neutral Postion (PNP) Switch Circuit Engine Torque Signal Circuit Problem

Repair Processes :

It should be pretty close to battery voltage (12 volts). If not, then this is the circuit that has the problem. Look for damage to the tank indicating impact that may have damaged the fuel pump or sender.

Process Tips

Diagnostic trouble code P1939 Subaru Stx is not difficult to diagnose and repair. Thus, it always pays to have a qualified technician to look after your vehicle should this error code arise.

Diagnosing this code requires the engine to be in good running shape, without unmetered air entering the engine. If other codes that relates to misfiring condition, lean running or air entering the engine is present, then you must first fix those error codes before P1939 Subaru Stx.

Fortunately, P1939 Subaru Stx is a fairly common OBD code, meaning that it applies to all makes and models of vehicles made after 1996.