Technical Description of OBD-II Trouble Code
The control circuit for Fuel Pressure Regulator 1 is experiencing low voltage.
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to OBD-II equipped vehicles (Isuzu, Mazda, Dodge, Chrysler, Ford, GMC, Chevy, Toyota, Honda, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
When a code P0091 is stored, it indicates that the powertrain control module (PCM) has detected a low voltage signal from the control circuit of the electronic fuel pressure regulator designated as 1. The numeral designation 1 is used in systems that have multiple electronic fuel pressure regulators. It may refer to a particular engine bank, but not always.
Typically, the PCM controls the electronic fuel pressure regulator. Battery voltage and ground signals are used to control a servo motor that positions the regulator valve to achieve the desired level of fuel pressure for any given situation. The fuel pressure sensor is located in the fuel injector rail and allows the PCM to monitor fuel pressure so that it can adjust the voltage of the fuel pressure regulator as necessary. Increasing voltage at the electronic fuel pressure regulator servo motor opens the valve and increases fuel pressure, while decreasing voltage causes the servo to retract and decreases fuel pressure.
Most often, the fuel pressure regulator and fuel pressure sensor are integrated into a single housing (with one electrical connector), but they may be separate components.
If the actual fuel pressure regulator control circuit voltage is lower than the desired level calculated by the PCM, a P0091 code will be stored and a malfunction indicator lamp (MIL) may illuminate.
Related engine codes for the fuel pressure regulator:
Symptoms & Severity
A code P0091 should be considered severe because excessive fuel pressure can lead to various drivability issues and cause damage to the internal engine and catalytic converter.
Symptoms of a P0091 code may include:
Causes
Possible causes for this code to be set are:
Diagnostic and Repair Procedures
To diagnose a code P0091, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), a suitable fuel pressure gauge, and a reliable vehicle information source (such as All Data DIY).
NOTE: Exercise extreme caution when using a fuel pressure gauge to avoid igniting high-pressure fuel on hot surfaces or open sparks, which could cause a fire.
Start by visually inspecting the system wiring and connectors, focusing on harnesses and connectors on top of the engine. Pests in cold climates often gnaw on system wiring and connectors in the warm upper engine area.
Connect the scanner to the vehicle diagnostic port and retrieve stored codes and freeze frame data. Write down this information as it will be helpful during the diagnostic process. Clear the codes and, if possible, test-drive the vehicle.
If the code is immediately reset, check for the correct voltage and battery ground at the fuel pressure regulator. If no voltage is found, test power supply relays and fuses following the wiring diagram from the vehicle information source. If there is no ground, use the wiring diagram to locate the fuel pressure regulator control system ground(s) and ensure they are secure.
If there is voltage and ground at the fuel pressure control regulator, obtain fuel pressure specifications from the vehicle information source and test the fuel system pressure using the fuel pressure gauge. Always follow the manufacturer’s recommendations for using the fuel pressure gauge. Use the scanner to observe fuel system data while visually monitoring manual fuel pressure with the fuel gauge.
If the level of fuel pressure reflected on the scanner does not match the actual fuel pressure, suspect a faulty fuel pressure sensor. Variations in fuel pressure regulator control voltage should correspond to fluctuations in actual fuel pressure. If this does not occur, suspect that either the fuel pressure regulator is defective, there is an open or shorted circuit in one of the fuel pressure regulator control circuits, or the PCM is faulty.
Use the DVOM and follow the manufacturer’s recommendations for testing the electronic fuel pressure regulator and the individual fuel pressure regulator control circuits. Before testing circuit resistance and continuity with the DVOM, disconnect controllers from the circuit to prevent damage to control modules.
Additional diagnostic notes:
Other fuel pressure trouble codes include:
