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This instruction show you guide on how to solve ISUZU 4JJ1 engine P0091 Fuel Pressure Regulator Control Circuit Low.

ISUZU IDSS MX2-T Diagnostic Tool

ISUZU E-IDSS 2024

P0091 DTC Information
1. DTC description

The FRP regulator is installed on the fuel supply pump and controls the fuel amount sucked into the common rail (fuel rail). The FRP regulator is fully opened in the normal state and larger drive current results in a smaller opening of the regulator. The ECM calculates the desired common rail (fuel rail) pressure and the fuel flow rate, and compares the calculated common rail (fuel rail) pressure to the actual pressure value to determine the opening position of the FRP regulator. If the actual common rail (fuel rail) pressure is higher than the desired pressure, the FRP regulator closes to decrease the fuel flow rate. If the ECM detects an excessively low feedback current in the FRP regulator, the DTC is set.

2. Condition for setting the DTC

Preconditions

The FRP regulator drive duty is 10% to 90%.

Setting conditions

The ECM detects that the feedback current of the FRP regulator is 50 mA or less, or lower than the desired current by 1,000 mA or more.

3. Action taken when DTC sets

Failure is indicated* in the monitor on the machine, or the diagnostic light illuminates.

Note: *Depending on the machine manufacturer, a failure may not be indicated.

The ECM limits the fuel injection quantity. The ECM limits the engine speed. The ECM stops multi-injection. The ECM stops the EGR control. The ECM stops DPD regeneration.

 

2.P0091 Diagnostics

Note: If the FRP regulator low-side circuit is shorted to ground, this DTC will not be set. It will cause the engine to stall or not to start.

1. OBD system check
 

2. Prioritized DTC check

1. Observe the DTC information with a trouble diagnosis scan tool. Is DTC P0092 set?

Yes

Go to DTC P0092 diagnosis.


NO

Go to Current DTC verification.

3. Current DTC check

1. Turn OFF the ignition switch for 30 seconds or more.

2. Turn ON the ignition switch.

3. Observe the DTC information with a trouble diagnosis scan tool. Is DTC P0091 set?

Yes

Go to FRP regulator low-side voltage check.

NO

Go to Intermittent conditions of engine.
 

4. FRP regulator low-side voltage check

1. Turn OFF the ignition switch.

2. Disconnect the FRP regulator harness connector.

3. Connect a test lamp between the FRP regulator valve low-side circuit and the frame ground.

4. Turn ON the ignition switch. Does the test lamp illuminate?

Yes

Go to Inspection for short to power supply in FRP regulator low-side circuit.

NO

Go to FRP regulator high-side voltage check.

5. FRP regulator high-side voltage check

1. Turn OFF the ignition switch.

2. Connect a test lamp between the FRP regulator valve high-side circuit and the frame ground.

3. Turn ON the ignition switch. Does the test lamp illuminate?

Yes

Go to FRP regulator low-side circuit inspection.

NO

Go to Inspection for open circuit and short circuit in FRP regulator high-side circuit.

6. FRP regulator low-side circuit inspection

1. Turn OFF the ignition switch.

2. Connect a test lamp between the FRP regulator low-side circuit and the battery power supply.

3. Turn ON the ignition switch. Does the test lamp illuminate?

Yes

Go to Inspection for short circuit in FRP regulator low-side circuit.

NO

Go to Inspection for open circuit in FRP regulator low-side circuit.

7. Inspection for short circuit in FRP regulator low-side circuit

1. Turn OFF the ignition switch with the test lamp still connected.

2. Disconnect the ECM harness connector. Does the test lamp illuminate?

Yes

Go to Inspection for short to ground in FRP regulator low-side circuit.

NO

Go to Inspection for short circuit in FRP regulator circuit.

8. Inspection for short to power supply in FRP regulator low-side circuit

1. Inspect the low-side circuit between the ECM and the FRP regulator for a short to the battery power supply circuit or a short to the ignition power supply circuit. Is the result normal?

Note: The ECM may be damaged if the FRP regulator low-side circuit is shorted to the power supply.

Yes

Go to ECM replacement.

NO

Repair the circuit as necessary.

Go to Repair verification.

9. Inspection for open circuit and short circuit in FRP regulator high-side circuit

Open circuit High resistance Short to ground

1. Inspect the high-side circuit between the ECM and the FRP regulator for the above conditions. Is the result normal?

Yes

Go to ECM harness connector inspection.

NO

Repair the circuit as necessary.

Go to Repair verification.

10. Inspection for open circuit in FRP regulator low-side circuit

1. Inspect the low-side circuit between the ECM and the FRP regulator for an open circuit or high resistance. Is the result normal?

Yes

Go to ECM harness connector inspection.

NO

Repair the circuit as necessary.

Go to Repair verification.

11. Inspection for short to ground in FRP regulator low-side circuit

1. Inspect the low-side circuit between the ECM and the FRP regulator for a short to ground. Is the result normal?

Yes

Go to ECM replacement.

NO

Repair the circuit as necessary.

Go to Repair verification.

12. Inspection for short circuit in FRP regulator circuit

1. Inspect for a short circuit between the FRP regulator high-side circuit and the low-side circuit. Is the result normal?

Yes

Go to FRP regulator harness connector inspection.

NO

Repair the circuit as necessary.

Go to Repair verification.

13. FRP regulator harness connector inspection

1. Inspect for poor connections at the FRP regulator harness connector. Is the connection status normal?

Yes

Replace the FRP regulator. 

Go to Repair verification.

NO

Repair the connections as necessary.

Go to Repair verification.

14. ECM harness connector inspection

1. Inspect for poor connections at the ECM harness connector. Is the connection status normal?

Yes
Go to ECM replacement.

NO
Repair the connections as necessary.
Go to Repair verification.

15. ECM replacement
Note: Perform programming after replacing the ECM. 
Procedure completion

Go to Repair verification.

16. Repair verification
1. Clear the DTC with a trouble diagnosis scan tool.
2. Turn OFF the ignition switch for 30 seconds or more.
3. Start the engine.
4. Perform a test-run under the conditions for running the DTC.

5. Observe the DTC information with a trouble diagnosis scan tool. A DTC should not be set.
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