P0091

DTC P0091

CIRCUIT DESCRIPTION

The fuel rail pressure (FRP) regulator is installed to the fuel supply pump and controls the suction fuel quantity into the fuel rail. The FRP regulator is fully opened in the normal state and larger drive current results in smaller opening. The engine control module (ECM) calculates desired fuel rail pressure and fuel flow rate and it compares the calculated desired fuel rail pressure to the actual value to determine the FRP regulator position. When the actual fuel rail pressure is higher than the desired value, the FRP regulator is closed to decease the flow rate. If the ECM detects an excessively low FRP regulator solenoid drive current, this DTC will set.

Condition for Running the DTC
- DTCs P0092, P0642 and P0643 are not set.
- The battery voltage is between 10-16 volts.
- The ignition switch is ON.
- The fuel supply pump control is stable.

Condition for Setting the DTC
- The ECM detects that the FRP regulator solenoid drive current is less than 400mA when the drive duty cycle is more than 70%.

Action Taken When the DTC Sets
- The ECM illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
- The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Freeze Frame/Failure Records.
- The ECM limits fuel rail pressure within 11600 psi (80 MPa).
- The ECM limits accelerator control range to idle.
- The ECM closes the EGR valve and holds to close position.
- The ECM inhibits the cruise control.
- The ECM inhibits the PTO control.

Condition for Clearing the MIL/DTC
- The ECM turns OFF the MIL after 3 consecutive driving cycles when the diagnostic runs and does not fail. (12,000 lbs GVW)
- The ECM turns OFF the MIL when the diagnostic runs and does not fail. (Except 12,000 lbs GVW)
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes after the key is cycled.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic after the ECM turns OFF the MIL.
- Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
If an intermittent condition is suspected, the following may cause an intermittent:
- Poor connections.
- Misrouted harness.
- Rubbed through wire insulation.
- Broken wire inside the insulation.

Check for the following conditions:
- Poor connection at the ECM: Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
- Damaged harness: Inspect the wiring harness for damage. If the harness appears to be OK, observe the "Actual FRP Regulator" display on the scan tool while moving connectors and wiring harness. A change in the display will indicate the location of the fault.

Test Description
The numbers below refers to the step number on the diagnostic table.
3. If the FRP regulator low control circuits between the ECM and the FRP regulator are short to battery or ignition voltage, this DTC may not set. DTC P0092 may set dependent upon location of a short.
9. If the FRP regulator low control circuits between the ECM and the FRP regulator are short to ground, this DTC may not set. This condition will cause engine stall or no engine start.

DTC P0091
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls Connector End Views or Engine Control Module (ECM) Connector End Views

Step 2 - 8:

Step 9 - 14:

Step 15 - 18: