How do I fix the code P0191?

When the P0191 code appears, it signals a problem with the fuel rail pressure sensor circuit or the sensor itself. In practice, diagnosing P0191 requires verifying both the sensor signal and the actual rail pressure. The typical fixes are replacing the sensor, repairing wiring, or addressing a fuel-system fault such as a failing pump or clogged filter.

What P0191 Means

P0191 stands for Fuel Rail Pressure Sensor Circuit Range/Performance. The engine computer expects the sensor signal to stay within a defined voltage range as engine load and RPM change. If the sensor signal is outside that range or the circuit has an issue (open, short, or poor reference/ground), the ECU stores this code and may set the check engine light. Causes can include a faulty sensor, damaged wiring or connector, a bad ground or 5-volt reference, or an actual abnormal fuel pressure due to mechanical faults in the fuel system.

Quick Diagnostic Checklist

Begin with broad checks to determine whether the problem is sensor-related or a fuel-system issue.

• Retrieve freeze-frame data and any related codes to understand when the fault occurred and engine conditions.


• Test actual fuel pressure with a mechanical gauge and compare it to the vehicle spec at idle and under load.


• Inspect the rail pressure sensor connector and wiring for corrosion, bent pins, damage, or loose connections.


• Look for obvious fuel leaks, damaged hoses, or a failing fuel pump or clogged fuel filter that could affect rail pressure.


• Scan live data for sensor voltage (5V reference, ground, and signal) and note any out-of-range readings.

These checks help determine whether the fault lies with the sensor/wiring or with the fuel-pressure supply.

Sensor and Wiring Diagnostic Steps

If the quick checks point toward the sensor or its wiring, perform targeted electrical tests while observing safety and using proper tools.

1. Back-probe the fuel rail pressure sensor wires with a multimeter or scanner to read voltage signals while the engine is running or at key on.


2. Verify 5V reference at the sensor pin from the ECU and confirm a solid ground connection to the sensor and ECU ground common with the engine block.


3. Check continuity of the signal wire from the sensor to the PCM/ECU; look for opens, high resistance, or shorts to ground or power.


4. Compare live sensor data to the expected range in the service manual; if the voltage does not track pressure changes, the sensor or signal wiring may be faulty.


5. Inspect the sensor’s electrical connector for corrosion, bent or pushed-in pins, and ensure the harness is not chafed or damaged by heat or moving parts.


6. For vehicles with known service bulletins, check for manufacturer recalls or TSBs addressing P0191 and update or replace as directed.

After these steps, you should be able to determine whether the fault is a sensor/wiring issue or if the problem lies elsewhere in the fuel system.

Key caveats for DIY inspections

Safety first: fuel systems can be hazardous. Relieve fuel pressure per the vehicle manual, work in a well-ventilated area, and avoid ignition sources. If you’re unsure, consult a professional technician.

Verifying Fuel Pressure vs Sensor Readings

To separate a sensor fault from an actual fuel-system problem, compare the sensor’s reading to the real rail pressure using a gauge and observe behavior under various loads and conditions.

1. Connect a certified fuel pressure gauge to the rail and record pressure at idle, under acceleration, and with the engine load high as per manual.


2. Check for discrepancies where the sensor shows a normal or abnormal reading that does not correspond with the measured rail pressure.


3. Inspect the vacuum line to the fuel pressure regulator (if applicable) for leaks or blockages, and verify regulator operation by observing pressure changes when vacuum is applied or released.


4. Evaluate the fuel pump performance: verify voltage to the pump, listen for a consistent pump operation, and consider testing the pump current draw if available.


5. If rail pressure is consistently out of spec but sensor data tracks pressure changes correctly, repair the fuel-pressure supply system rather than replacing the sensor.

Accurate diagnosis requires correlating sensor data with actual pressure measurements and system performance.

Common Fixes

Based on the diagnostic outcomes, apply the appropriate repair, prioritizing safety and using OEM-recommended parts and procedures.

• Replace a faulty fuel rail pressure sensor when electrical tests indicate a defective device or the sensor signal does not respond to pressure changes.


• Repair or replace damaged wiring or connectors, including re-seating or replacing corroded pins and ensuring proper grounding.


• Address fuel-system hardware faults: replace a failing fuel pump or faulty regulator, replace clogged fuel filters, or repair leaks in the return or supply lines.


• Ensure vacuum lines and connections to any fuel-pressure regulator are intact and free of leaks.


• Clear codes, perform drive cycles, and re-check data to confirm the issue is resolved. Check for related codes that may appear after the fix.

With the root cause addressed, the P0191 code should clear and the engine should operate normally if no further issues arise.

Summary

P0191 signals a problem with the fuel rail pressure sensor circuit range or performance. A careful approach starts with a quick diagnostic checklist to distinguish sensor/wiring from fuel-system faults, followed by targeted electrical tests and fuel-pressure verification. Replacing a faulty sensor or repairing wiring is common, but if rail pressure is out of spec due to pump, regulator, or leaks, those components must be addressed. Always consult the vehicle's service manual for exact specifications and procedures, and prioritize safety when working near fuel systems.

What would cause a P0191 code?

One of the most common reasons why a vehicle logs a P0191 code is due to a malfunctioning fuel pressure sensor. Replacing the fuel pressure sensor should clear the code, but you'll need to find a high-quality replacement to ensure the problem is fixed for good.

Can a clogged fuel filter cause a P0191 code?

Common Causes of P0191 in Ford
Clogged Fuel Filter: Dirt and debris in a clogged filter block fuel flow, lowering rail pressure. Malfunctioning Fuel Pressure Regulator: This component fails to adjust fuel pressure based on engine needs. Leaky Fuel Lines: Leaks let fuel escape, reducing pressure in the rail.

How to fix fuel rail system pressure too low?

To fix no fuel pressure, start by checking the fuel pump fuse and relay, followed by the fuel filter. If those are functional, inspect the fuel pump for blockages or issues, check the fuel pressure regulator, look for leaks, and ensure all electrical connections are secure. 
Before you begin

• Safety first: Wear safety glasses and gloves, as fuel is flammable and can be harmful. Relieve any residual fuel pressure before disconnecting any lines. 
• Gather tools: You will need a fuel pressure gauge, a multimeter, and basic hand tools. 

Troubleshooting steps
This video explains how to diagnose and fix common issues with fuel pressure: 1mMechanic BaseYouTube · Apr 15, 2024

1. Check the fuse and relay: 
   • Locate the fuse box and check if the fuel pump fuse is blown; replace it if it is. 
   • Swap the fuel pump relay with a similar, working relay (like the horn or A/C relay) to see if that solves the problem. If the car starts, the old relay was bad. 
2. Inspect the fuel filter: 
   • A clogged fuel filter is a common cause of low or no fuel pressure. 
   • If the filter is easily accessible, replace it. If it's inside the fuel tank, it will be more involved to replace. 
3. Test the fuel pump circuit: 
   • Use a multimeter to check for voltage at the fuel pump connector while the ignition is on. 
   • If there is power but no fuel pressure, the pump may be faulty or the in-tank filter/strainer is clogged. 
   • If there is no power, there is an electrical issue with the wiring, fuse, or relay. 
4. Examine the fuel pump: 
   • If the pump is getting power and the filter is clean, the pump itself could be bad. You can sometimes remove the pump assembly and tap the motor to see if it frees up, but it may need replacement. 
   • Check for a faulty check valve on the pump, which can cause pressure to bleed off after shutdown. 
5. Check the fuel pressure regulator: 
   • A faulty regulator can leak fuel back to the tank, causing low pressure. 
   • Inspect the vacuum hose connected to the regulator for cracks or leaks. Replace the regulator if it's damaged or clogged. 

This video demonstrates how to diagnose and replace a faulty fuel pressure regulator: 59sPhillips VisionYouTube · Nov 23, 2021

1. Inspect the fuel rail and lines:
   • Check for any physical clogs or damage in the fuel rail itself.
   • Look for leaks from the fuel rail, lines, or injectors. A soft or collapsed rubber fuel line can also be the cause. 

How do I reset the fuel pressure sensor?

You cannot "reset" a fuel pressure sensor in the traditional sense; instead, you must address the underlying issue. If a fault code is present, clearing it with a scan tool may be necessary after a fix, but often the only way to resolve a sensor problem is to replace the sensor itself. For issues like a fuel pump cut-off, you can look for a fuel pump reset button, often located under the dashboard, and push it to reset the system. 
You can watch this video to learn how to reset a fuel pump shut off switch: 48sAlley 55 CustomsYouTube · Jul 16, 2024
For a faulty fuel pressure sensor 

• Diagnose the problem: A faulty sensor will likely trigger a check engine light. You can use a multimeter to test the sensor's functionality before replacing it by checking for proper voltage and signal output. 
• Replace the sensor: If the sensor is bad, the only way to fix it is to replace it. This is often a straightforward task you can do yourself or have a mechanic do. 
  • Relieve fuel pressure first by waiting for the engine to cool or by following the manufacturer's specific procedure. 
  • Disconnect the electrical connector and any vacuum lines. 
  • Remove the old sensor and install the new one. 
  • Reconnect everything and start the engine to check for proper operation. 
• Clear the fault code: After replacing the sensor, use a scan tool to clear any remaining fault codes in the vehicle's computer. 

This video demonstrates how to replace a fuel rail pressure sensor: 39sDanny Johnson’s GarageYouTube · Mar 18, 2022
For a fuel pump that won't start

• Locate the reset button: Many vehicles have a safety feature called an inertia switch that cuts power to the fuel pump after an impact. 
• Reset the switch: Press the button to reset the system. The button is often located under the dashboard, but consult your owner's manual for its exact location. 
• Check for other issues: If the reset button doesn't fix the problem, there might be other issues, such as a faulty fuel pump relay. In this case, you may need to remove and reinstall the relay or check other fuses. 

You can watch this video to learn about symptoms of a bad fuel pressure sensor: 46sEasyAutoFixYouTube · Jan 21, 2023